Always use a single GdkDisplay

This uses mutter!2810 (merged) to get access to a privileged private Wayland protocol used for mapping Wayland dialogs on top of X11 clients, thus meaning we can stop opening an X11 connection in a Wayland session. That means no more cairo forcing, thus performant file chooser dialogs.

This also means the Wayland connection is opened via a D-Bus API and not the regular general purpose Wayland socket.