Add an option to run as a wayland compositor

Move the option from the mutter binary to libmutter, and
rename from --nested to --wayland.
This allows gnome-shell to drop the set_is_wayland_compositor()
call.

https://bugzilla.gnome.org/show_bug.cgi?id=707897
Fix regression in mutter-wayland translations

Switching meta/util.h to gi18n.h was wrong, mutter is a library
and needs gi18n-lib.h, but that cannot be included from a public
header (since it depends on config.h or command line options),
so split util.h into a public and a private part.

https://bugzilla.gnome.org/show_bug.cgi?id=707897
Don't translate debug logs and warnings

Warnings that are going to the journal should be not translated:
they're not user visible, and translating them would just make
bug reporting harder (as now the developers need to understand
what the warning is saying)

https://bugzilla.gnome.org/show_bug.cgi?id=707897