wayland: Implement on-demand start of Xwayland

The Xwayland manager now has 4 distinct phases:
- Init and shutdown (Happening together with the compositor itself)
- Start and stop

In these last 2 phases, handle orderly initialization and shutdown
of Xwayland. On initialization We will simply find out what is a
proper display name, and set up the envvar and socket so that clients
think there is a X server.

Whenever we detect data on this socket, we enter the start phase
that will launch Xwayland, and plunge the socket directly to it.
In this phase we now also set up the MetaX11Display.

The stop phase is pretty much the opposite, we will shutdown the
MetaX11Display and all related data, terminate the Xwayland
process, and restore the listening sockets. This phase happens
on a timeout whenever the last known X11 MetaWindow is gone. If no
new X clients come back in this timeout, the X server will be
eventually terminated.

The shutdown phase happens on compositor shutdown and is completely
uninteresting. Some bits there moved into the stop phase as might
happen over and over.

This is all controlled by META_DISPLAY_POLICY_ON_DEMAND and
the "autostart-xwayland" experimental setting.

GNOME/mutter!709