compositor: Add XWayland basic support

compositor: Add XWayland basic support

This allow starting up XWayland and the ability to display, albeit
is incomplete, given that we have no way at this moment to make the
window maximized, as we do with xdg-shell/native wayland applications.

A further patch, together with libweston changes are needed to make this
complete. For now this can provide us with initial smoke testing. Tested
with basic X11 clients, including Unigine, which is the reason for
having this the moment.

Note that this at the moment compiled-out, requiring the pass
-Dxwayland=true when building the compositor.

Bug-AGL: SPEC-4782
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I87d2a852165cb3f03482bea1e04931bdd6d4c115