Allow auto-shutdown of Xwayland
We will need a few things for this, current idea is:
- Start gsd daemons using systemd user session. Mutter could e.g. start
xwayland-services.unit
that we depend on - We need to split all modules that need to run on wayland but use X11 into two modules (or have one common X11 module). Example: color plugin to set a root window property