Remove Mutter namespace prefix

Move all objects and functions namespaced with Mutter into the Meta namespace
to get a single consistent namespace. Changes that aren't simply changing mutter
to meta:

 MutterWindow              => MetaWindowActor
 mutter_get_windows        => meta_get_window_actors
 mutter_plugin_get_windows => meta_plugin_get_window_actors

https://bugzilla.gnome.org/show_bug.cgi?id=628520