keybindings: Add is_builtin() method

https://bugzilla.gnome.org/show_bug.cgi?id=688202

src/core/keybindings.c

@@ -95,6 +95,12 @@ meta_key_binding_get_mask (MetaKeyBinding *binding)
   return binding->mask;
 }
 
+gboolean
+meta_key_binding_is_builtin (MetaKeyBinding *binding)
+{
+  return binding->handler->flags & META_KEY_BINDING_BUILTIN;
+}
+
 /* These can't be bound to anything, but they are used to handle
  * various other events.  TODO: Possibly we should include them as event
  * handler functions and have some kind of flag to say they're unbindable.

src/meta/keybindings.h

@@ -28,6 +28,7 @@
 const char          *meta_key_binding_get_name      (MetaKeyBinding *binding);
 MetaVirtualModifier  meta_key_binding_get_modifiers (MetaKeyBinding *binding);
 guint                meta_key_binding_get_mask      (MetaKeyBinding *binding);
+gboolean             meta_key_binding_is_builtin    (MetaKeyBinding *binding);
 
 gboolean meta_keybindings_set_custom_handler (const gchar        *name,
 					      MetaKeyHandlerFunc  handler,