app: add all our actions in the GApplication.

Right now, our actions are added both in the GActionMap (a.k.a. as
GApplication|GtkApplication) and with the old GtkActionGroup API. The later code
is meant to be removed soon.

A major difference with the old implementation is that we don't add the actions
in separate action groups. Even though GLib has a GActionGroup concept, these
don't seem to be used at all when adding actions on the GtkApplication level.
It might mean that eventually we will even remove the GimpActionGroup code and
move everything up one level.

Of course, I am still evaluating if we really want an action group concept, in
which case, we could simply recreate it, but I'm actually unsure how useful it
is. The only place where I see a use to this is in the shortcut settings (to
organize actions by groups), but even there, it was rarely useful to me. I
usually rather search for actions by text search anyway.

app/app.c

@@ -247,6 +247,8 @@ app_run (const gchar         *full_prog_name,
   app = gimp_console_app_new (gimp, quit, as_new, filenames, batch_interpreter, batch_commands);
 #endif
 
+  gimp->app = app;
+
   gimp_cpu_accel_set_use (use_cpu_accel);
 
   /*  Check if the user's gimp_directory exists */

app/core/gimp.h

@@ -36,6 +36,8 @@ struct _Gimp
 {
   GimpObject              parent_instance;
 
+  GApplication           *app;
+
   GimpCoreConfig         *config;
   GimpCoreConfig         *edit_config; /* don't use this one, it's just
                                         * for the preferences dialog

app/widgets/gimpactiongroup.c

@@ -326,6 +326,24 @@ gimp_action_group_add_action_with_accel (GimpActionGroup *action_group,
                                          GimpAction      *action,
                                          const gchar     *accelerator)
 {
+
+  /* Making sure all our Gimp*Action classes are also GAction. */
+  g_return_if_fail (G_IS_ACTION (action));
+
+  g_action_map_add_action (G_ACTION_MAP (action_group->gimp->app), G_ACTION (action));
+  if ((accelerator != NULL && g_strcmp0 (accelerator, "") != 0))
+    {
+      gchar* detailed_action_name;
+
+      detailed_action_name = g_strdup_printf ("app.%s",
+                                              g_action_get_name (G_ACTION (action)));
+      gtk_application_set_accels_for_action (GTK_APPLICATION (action_group->gimp->app),
+                                             detailed_action_name,
+                                             (const char*[]) { accelerator, NULL });
+      g_free (detailed_action_name);
+    }
+
+  /* TODO: remove the old logic with GtkAction. */
   gtk_action_group_add_action_with_accel ((GtkActionGroup *) action_group,
                                           (GtkAction *) action,
                                           accelerator);