Rework and consolidate monitor handling in MetaScreen

Consolidate all places that deal with output configuration in
MetaScreen, which gets it either from XRandR or from a dummy static configuration.
We still need to read the Xinerama config, even when running xwayland,
because we need the indices for _NET_WM_FULLSCREEN_MONITORS, but
now we do it only when needed.

https://bugzilla.gnome.org/show_bug.cgi?id=705670