Add private API for overriding the compositor configuration, i.e. the
compositor type (X11 WM or Wayland compositor) and backend type. This
will make it possible to add a special test backend used by src/tests/.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
Introduce two new clutter backends: MetaClutterBackendX11 and
MetaClutterBackendNative. They are so far only wrap ClutterBackendX11
and ClutterBackendEglNative respectively, but the aim is to move things
from the original clutter backends when needed.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
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
