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

src/core/window-private.h

@@ -165,7 +165,7 @@ struct _MetaWindow
    * been overridden (via a client message), the window will cover the union of
    * these monitors.  If not, this is the single monitor which the window's
    * origin is on. */
-  long fullscreen_monitors[4];
+  gint fullscreen_monitors[4];
   
   /* Whether we're trying to constrain the window to be fully onscreen */
   guint require_fully_onscreen : 1;