barriers: Separate implementation from public API

This patch removes the X11 specific code from MetaBarrier and creates an
abstraction layer MetaBarrierImpl. The existing X11 implementation is
moved to a new GObject MetaBarrierImplX11 implementing the abstract
interface MetaBarrierImpl which is instantiated by MetaBarrier when
supported.

While at it, move it to backends/ and properly name the files.

https://bugzilla.gnome.org/show_bug.cgi?id=706655