MonitorManager: split the XRandR parts in a subclass

Instead of keeping a forest of if backend else ..., use a subclass
and virtual functions to discriminate between XRandR and the
dummy backend (which lives in the parent class togheter with the
common code)

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

src/core/screen.c

@@ -670,7 +670,7 @@ meta_screen_new (MetaDisplay *display,
   screen->xroot = xroot;
   screen->rect.x = screen->rect.y = 0;
   
-  meta_monitor_manager_initialize (screen->display->xdisplay);
+  meta_monitor_manager_initialize ();
 
   manager = meta_monitor_manager_get ();
   g_signal_connect (manager, "monitors-changed",