keybindings: Allow to add/remove keybindings at runtime

Add meta_display_add_keybinding()/meta_display_remove_keybinding(),
which allow to add/remove keybindings dynamically at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=663428

src/core/keybindings-private.h

@@ -77,6 +77,9 @@ gboolean meta_prefs_add_keybinding          (const char           *name,
                                              MetaKeyBindingAction  action,
                                              MetaKeyBindingFlags   flags);
 
+gboolean meta_prefs_remove_keybinding       (const char    *name);
+
+
 #endif