plug-ins: adding a hello-world plug-in in javascript.

It is not installed yet, I am just committing my first version. Further
version should look like goat-exercise as an example of what must be
done in javascript.
Also the plug-in stopped working with one of the recent libgimp commits,
without any change in the plug-in code. This will need to be
investigated before installing.

plug-ins/goat-exercises/hello-world.js

@@ -0,0 +1,69 @@
+#!/usr/bin/gjs
+
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * hello-world.js
+ * Copyright (C) Jehan
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+const System = imports.system
+
+imports.gi.versions.Gimp = '3.0';
+const Gimp = imports.gi.Gimp;
+imports.gi.versions.Gtk = '3.0';
+const Gtk = imports.gi.Gtk;
+
+const GLib = imports.gi.GLib;
+const GObject = imports.gi.GObject;
+
+/* gjs's ARGV is not C-style. We must add the program name as first
+ * value.
+ */
+ARGV.unshift(System.programInvocationName);
+
+var MyPlugIn = GObject.registerClass({
+    GTypeName: 'GimpMyPlugin',
+}, class MyPlugIn extends Gimp.PlugIn {
+
+    vfunc_query_procedures() {
+        return ["javascript-fu-hello-world"];
+    }
+
+    vfunc_create_procedure(name) {
+        let procedure = Gimp.Procedure.new(this, name, Gimp.PDBProcType.PLUGIN, this.run);
+        procedure.set_menu_label("Hello World in GJS")
+        procedure.set_documentation("Hello World in GJS",
+                                    "Create a new image to demonstrate a javascript plug-in.",
+                                    "")
+        procedure.set_attribution("Jehan", "Jehan", "2019")
+        /*procedure.add_menu_path ('<Image>/Filters/Hello World')*/
+
+        return procedure;
+    }
+
+    run(procedure, args, data) {
+        let image = Gimp.image_new(1000, 1000, Gimp.ImageBaseType.RGB);
+        let layer = Gimp.layer_new(image, "Hello World",
+                                   1000, 1000, Gimp.ImageType.RGB_IMAGE,
+                                   100.0, Gimp.LayerMode.NORMAL);
+        Gimp.image_insert_layer(image, layer, null, 0);
+        Gimp.display_new(image);
+        return procedure.new_return_values(Gimp.PDBStatusType.SUCCESS, null)
+    }
+});
+
+Gimp.main(MyPlugIn.$gtype, ARGV)