util: fix warnings when compiling gnome-shell-wayland

Replace our version of the gettext macros with those in glib,
so that they don't conflict with gnome-shell.

https://bugzilla.gnome.org/show_bug.cgi?id=707851
Fix regression in mutter 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

Conflicts:
	src/compositor/compositor.c
	src/meta/util.h