build: remove flathub's submodule and update flatpak howto.

Now our nightly manifest does not depend on the base module used in
flathub so just remove the submodule.
Also update our howto.

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "build/flatpak/flathub"]
-	path = build/flatpak/flathub
-	url = https://github.com/flathub/org.gimp.GIMP.git

build/flatpak/flathub

@@ -1 +0,0 @@
-Subproject commit b6030b22367300babf82d5ed3b9158d25c9902f0

build/flatpak/flatpak-howto.txt

@@ -5,8 +5,6 @@ Stable build
 ~~~~~~~~~~~~
 
 The stable manifest is maintained at flathub: https://github.com/flathub/org.gimp.GIMP
-This repository is pulled in GIMP repository as a submodule so that we
-can share as much code as possible with the other builds.
 
 To prepare a new build:
 
@@ -36,28 +34,59 @@ Development and nightly builds
 Flathub does not host non-stable builds, therefore they are maintained
 separately on the GIMP repository. Nevertheless they should remain as
 close to the stable manifest as possible since they are meant to become
-stable too.
+stable too eventually.
 
-Building and maintaining
-~~~~~~~~~~~~~~~~~~~~~~~~
+Building
+--------
 
-* Init the submodule if this is the first time:
+* Dependencies:
+  - flatpak (at least 0.9.5)
+  - flatpak-builder (at least 0.9.5)
+  - appstream-compose
+  - a GPG key to sign the builds
 
-> git submodule init
-
-* Update it:
-
-> git submodule update
-
-* Install flatpak and flatpak-builder. There are packages for most
+Note: there are packages of `flatpak` and `flatpak-builder` for most
       distributions. Check: http://flatpak.org/getting.html
-  Use at least flatpak 0.9.5.
-
-* Make sure `appstream-compose` is installed as well. This is used to parse the
-  appdata file and generate the appstream (metadata like comments, etc.).
-  For instance on Fedora, this is provided by the package `libappstream-glib`,
+Note 2: `appstream-compose` is used to parse the appdata file and
+        generate the appstream (metadata like comments, etc.).
+        On Fedora, this is provided by the package `libappstream-glib`,
         on Ubuntu by `appstream-util`…
 
+* Install the runtimes and the corresponding SDKs if you haven't already:
+
+> flatpak remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo
+> flatpak install gnome org.gnome.Platform/x86_64/3.28 org.gnome.Sdk/x86_64/3.28
+> flatpak install gnome org.gnome.Platform/i386/3.28 org.gnome.Sdk/i386/3.28
+> flatpak install gnome org.gnome.Platform/arm/3.28 org.gnome.Sdk/arm/3.28
+> flatpak install gnome org.gnome.Platform/aarch64/3.28 org.gnome.Sdk/aarch64/3.28
+
+Or simply update them if you have already installed them:
+
+> flatpak update
+
+* Run the build script:
+
+> ./gimp-flatpak-cron.sh <GPGKEY> <REPO> <LOGDIR> <BUILDDIR>
+
+Note that it is named with the "-cron" suffix because it is a good cron
+script for nightlies.
+
+The parameters:
+
+GPGKEY: your GPG key used to sign the builds
+REPO: the directory where the repository is locally.
+      For instance, it could be /var/www/html/flatpak/ (which could map
+      to https://download.gimp.org/flatpak/ for instance)
+LOGDIR: where the logs of the build will be kept (under subdirectories
+        by date).
+BUILDDIR: where the built prefix will be kept for later review.
+
+The export commands will output a commit hash. Save it for further
+verification.
+
+Maintaining the manifests
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
 * GIMP uses Flatpak's GNOME runtime, which contains a base of libraries,
   some of which are dependencies of GIMP.
   Check out the available versions at: http://flatpak.org/runtimes.html
@@ -65,23 +94,11 @@ Building and maintaining
 
 > "runtime-version": "3.28"
 
-* Install this runtime and the corresponding SDK if you haven't already:
-
-> flatpak remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo
-> flatpak install gnome org.gnome.Platform/x86_64/3.28 org.gnome.Sdk/x86_64/3.28
-> flatpak install gnome org.gnome.Platform/i386/3.28 org.gnome.Sdk/i386/3.28
-> flatpak install gnome org.gnome.Platform/arm/3.28 org.gnome.Sdk/arm/3.28
-> flatpak install gnome org.gnome.Platform/aarch64/3.28 org.gnome.Sdk/aarch64/3.28
-
-Or simply update them:
-
-> flatpak update
-
 * Other GIMP dependencies which are not available in the GNOME runtime
   should be built along as modules within GIMP's flatpak.
-  Check format in `org.gimp.GIMP.json` and add modules if necessary.
-  For more information and options, check flatpak builder's manifest
-  format:
+  Check format in `org.gimp.GIMP-nightly.json` and add modules if
+  necessary.  For more information and options, check flatpak builder's
+  manifest format:
   http://flatpak.org/flatpak/flatpak-docs.html#flatpak-builder
 
 * On the other hand, if we increased the runtime version in particular,
@@ -98,7 +115,7 @@ Or simply update them:
   Other than by trial and error, you can find the installed dependencies
   by running:
 
-> flatpak run --devel --command=bash org.gnome.Sdk//3.24
+> flatpak run --devel --command=bash org.gnome.Sdk//3.28
 
 Or if you already have a build:
 
@@ -136,35 +153,6 @@ Or if you already have a build:
   "gimp", "babl" and "gegl" modules, set "branch" to "master", and
   remove any "commit" line.
 
-* Run flatpak builder with the following commands:
-
-> flatpak-builder --ccache --arch=x86_64 gimp-flatpak-build-x86-64 org.gimp.GIMP.json
-> flatpak-builder --ccache --arch=i386 gimp-flatpak-build-i386 org.gimp.GIMP.json
-
-  Note: if you ever want to check the `config.log` of any of the module which
-  failed to compile, it is available in a hidden directory of your working path.
-  For instance GIMP build dir will be `.flatpak-builder/build/gimp-1/`.
-  The number will be incrementing at each build.
-  If you want to check build directories even if the builds are successful, add
-  `--keep-build-dirs` option to your command.
-
-  If it ends successfully, this will have built GIMP in new folders,
-  `gimp-flatpak-build-x86-64/` and `gimp-flatpak-build-i386/`
-  respectively.
-
-* Export the 2 builds:
-
-> flatpak-builder --export-only --gpg-sign=YOUR_GPG_KEY --repo=/path/to/repo --arch=x86_64 gimp-flatpak-build-x86-64 org.gimp.GIMP.json
-> flatpak-builder --export-only --gpg-sign=YOUR_GPG_KEY --repo=/path/to/repo --arch=i386 gimp-flatpak-build-i386 org.gimp.GIMP.json
-
-  If you key was protected with a passphrase, you will need to input it.
-  The same repository will contain all the builds.
-  Note: do not use `flatpak build-export` which is an internal low-level
-  command. See https://github.com/flatpak/flatpak/issues/824
-
-  The export commands will output a commit hash. Save it for further
-  verification.
-
 * Test it locally with:
 
 > flatpak --user remote-add --no-gpg-verify gimp /path/to/repo
@@ -187,14 +175,3 @@ Or remotely:
   Also an entry "(Dev) GNU Image Manipulation Software" should now be
   available in your menus (or GNOME Overview). This will run the flatpak
   build.
-
-* TODO: this is my current state. I still have to probably write a
-  `.flatpakrepo` file which we can upload on `gimp.org` and should allow
-  GNU/Linux users to install GIMP and our flatpak repo in a few clicks.
-  We will also have to sign the repo with GPG, etc.
-
-* TODO: Windows builds are defined in: build/windows/jhbuild/build.jhbuildrc
-  It would be a good idea to compare contents and maybe deactivate some
-  useless options from dependencies. Also if all builds end up having a
-  very similar dependency list, maybe there could be a meta-format from
-  which both the jhbuild and the flatpak manifest could be generated.