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/data-editor-commands.c

@@ -23,7 +23,6 @@
 #include "actions-types.h"
 
 #include "widgets/gimpdataeditor.h"
-#include "widgets/gimptoggleaction.h"
 
 #include "data-editor-commands.h"
 
@@ -32,12 +31,13 @@
 
 void
 data_editor_edit_active_cmd_callback (GimpAction *action,
+                                      GVariant   *value,
                                       gpointer    data)
 {
   GimpDataEditor *editor = GIMP_DATA_EDITOR (data);
   gboolean        edit_active;
 
-  edit_active = gimp_toggle_action_get_active (GIMP_TOGGLE_ACTION (action));
+  edit_active = g_variant_get_boolean (value);
 
   gimp_data_editor_set_edit_active (editor, edit_active);
 }