widget: add helpers to resolve GActionGroups available to GtkWidget

These functions, while added for use by the GTK inspector, are generally
useful to applications that need to resolve what action groups are
available to a particular GtkWidget.

https://bugzilla.gnome.org/show_bug.cgi?id=741386

gtk/gtkactionmuxer.c

@@ -732,10 +732,10 @@ gtk_action_muxer_remove (GtkActionMuxer *muxer,
     }
 }
 
-gchar **
+const gchar **
 gtk_action_muxer_list_prefixes (GtkActionMuxer *muxer)
 {
-  return (gchar **) g_hash_table_get_keys_as_array (muxer->groups, NULL);
+  return (const gchar **) g_hash_table_get_keys_as_array (muxer->groups, NULL);
 }
 
 GActionGroup *