This completes my earlier commit 406279e4ef.
Extension installation is not about just decompressing a file in the
right folder. We must also make the extension manager and the GUI aware
of this newly available extension.
File extension (.gex) may still change if anything better is proposed.
This format is currently just a compressed archive containing the
extension data (plug-in, brushes or whatever) and the metadata file.
For now, opening such file will simply install it as a new extension,
keeping all file permissions and structure. Of course in the future, it
will have to trigger a confirmation dialog.
Currently the compression used is zip, which is just a first draft. This
is not a decisive choice as well. We could use some tarball compressed
in whatever other compression algorithm. I use libarchive as a new
dependency to support unarchiving as it seems to be a common library
(and since it is already used by AppStream-glib anyway, this doesn't add
any actual dependency, just make it direct).
Michael Natterer