Send _NET_WM_FRAME_DRAWN messages

When the application provides the extended second counter for
_NET_WM_SYNC_REQUEST, send a client message with completion
information after the next redraw after each counter update
by the application.

https://bugzilla.gnome.org/show_bug.cgi?id=685463

src/meta/atomnames.h

@@ -174,6 +174,7 @@ item(_NET_WM_FULLSCREEN_MONITORS)
 item(_NET_WM_STATE_FOCUSED)
 item(_NET_WM_BYPASS_COMPOSITOR)
 item(_NET_WM_OPAQUE_REGION)
+item(_NET_WM_FRAME_DRAWN)
 
 #if 0
 /* We apparently never use: */