Initial X11 support

XWayland seems a bit unstable (as of now it freezes when an X11
application is launched), this makes integration with the system
compositor harder.

A lot of people still relies on X11, in order to get a wider user base
we need to support X11 while keeping the Wayland support active.

Issue: #11
Run Xorg server (or System Compositor) and greeter

When a seat starts we run a Xorg server (or a System Compositor if X11
support is disabled) and then the greeter.

Issue: #11