Extend the DBus XRandR protocol to expose cloning restriction

Turns out that even if two outputs say that they can be controlled
by a given CRTC, you can't configure them in the same CRTC unless
they are marked as "possible clones" one of the other.
This can further restrict the configuration options, so we need
to expose this limitation in the DBus API.

src/core/monitor-private.h

@@ -70,6 +70,9 @@ struct _MetaOutput
   MetaCRTC **possible_crtcs;
   unsigned int n_possible_crtcs;
 
+  MetaOutput **possible_clones;
+  unsigned int n_possible_clones;
+
   /* The low-level bits used to build the high-level info
      in MetaMonitorInfo