clutter: Reuse GTK+'s RTL/LTR handling

Commit a4fb7ef5 dropped translations of our internal cogl/clutter
forks, which broke the local-based text direction support. Instead
of bringing back translations just for this purpose, we can re-use
GTK's translations which use the same technique.

https://bugzilla.gnome.org/show_bug.cgi?id=771549