DisplayConfig: make the dummy backend writable

Add a number of dummy outputs and modes to the dummy backend,
and implement the writing bits.
The only visible effect is that you can change the screen size,
which resizes the output window.

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

src/core/monitor-private.h

@@ -73,6 +73,9 @@ struct _MetaOutput
   MetaOutput **possible_clones;
   unsigned int n_possible_clones;
 
+  /* Used when changing configuration */
+  gboolean dirty;
+
   /* The low-level bits used to build the high-level info
      in MetaMonitorInfo