sink: Implement GstNavigation interface
sink: Implement GstNavigation interface

So that DVD menus work.

Fixes: http://bugzilla.clutter-project.org/show_bug.cgi?id=2624

ystreet/clutter-gst,GNOME/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,ystreet/clutter-gst,skinkie/clutter-gst,skinkie/clutter-gst,GNOME/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,skinkie/clutter-gst,GNOME/clutter-gst,lubosz/clutter-gst,GNOME/clutter-gst