app: GtkAction -> GAction madness part two

Change all action callbacks so they can be invoked by a GAction:

- add GimpActionCallback typedef:
  void (* cb) (GimpAction*, GVariant*, gpointer)
- change all action callbacks to the GimpActionCallback signature
- add "gimp-activate" and "gimp-change-state" signals to GimpAction,
  with the same signature as the resp. GAction signals
- remove all other custom action signals and only use the new
  GimpAction signals
- pass around appropriate GVariants containing booleans, int32,
  strings
- badly hack around to force a GimpProcedure pointer into a
  uint64 variant
- remove all G_CALLBACK() casts from all action callbacks,
  they all have the same signature now

(cherry picked from commit 3b6b3fc189)

app/actions/debug-commands.c

@@ -73,6 +73,7 @@ static gboolean  debug_accel_find_func         (GtkAccelKey *key,
 
 void
 debug_mem_profile_cmd_callback (GimpAction *action,
+                                GVariant   *value,
                                 gpointer    data)
 {
   extern gboolean  gimp_debug_memsize;
@@ -88,6 +89,7 @@ debug_mem_profile_cmd_callback (GimpAction *action,
 
 void
 debug_benchmark_projection_cmd_callback (GimpAction *action,
+                                         GVariant   *value,
                                          gpointer    data)
 {
   GimpDisplay *display;
@@ -98,6 +100,7 @@ debug_benchmark_projection_cmd_callback (GimpAction *action,
 
 void
 debug_show_image_graph_cmd_callback (GimpAction *action,
+                                     GVariant   *value,
                                      gpointer    data)
 {
   GimpImage *source_image = NULL;
@@ -108,6 +111,7 @@ debug_show_image_graph_cmd_callback (GimpAction *action,
 
 void
 debug_dump_menus_cmd_callback (GimpAction *action,
+                               GVariant   *value,
                                gpointer    data)
 {
   GList *list;
@@ -149,6 +153,7 @@ debug_dump_menus_cmd_callback (GimpAction *action,
 
 void
 debug_dump_managers_cmd_callback (GimpAction *action,
+                                  GVariant   *value,
                                   gpointer    data)
 {
   GList *list;
@@ -177,6 +182,7 @@ debug_dump_managers_cmd_callback (GimpAction *action,
 
 void
 debug_dump_keyboard_shortcuts_cmd_callback (GimpAction *action,
+                                            GVariant   *value,
                                             gpointer    data)
 {
   GimpDisplay      *display;
@@ -266,6 +272,7 @@ debug_dump_keyboard_shortcuts_cmd_callback (GimpAction *action,
 
 void
 debug_dump_attached_data_cmd_callback (GimpAction *action,
+                                       GVariant   *value,
                                        gpointer    data)
 {
   Gimp        *gimp         = action_data_get_gimp (data);