Make MetaDisplay a real GObject

We need this to be able to add signals, among other things.

http://bugzilla.gnome.org/show_bug.cgi?id=563047

src/core/display-private.h

@@ -74,6 +74,8 @@ typedef void (* MetaWindowPingFunc) (MetaDisplay *display,
 
 struct _MetaDisplay
 {
+  GObject parent_instance;
+  
   char *name;
   Display *xdisplay;
 
@@ -298,6 +300,11 @@ struct _MetaDisplay
 #endif
 };
 
+struct _MetaDisplayClass
+{
+  GObjectClass parent_class;
+};
+
 /* Xserver time can wraparound, thus comparing two timestamps needs to take
  * this into account.  Here's a little macro to help out.  If no wraparound
  * has occurred, this is equivalent to