Dispose of cheese widget on avatar dialog destroy

This ensures that the cheese widget is destroyed when the avatar dialog
is closed. If the widget is not destroyed then the camera device remains
in use, causing a blank preview to be shown if the dialog is reopened.

https://bugzilla.gnome.org/show_bug.cgi?id=700959

src/contacts-avatar-dialog.vala

@@ -448,6 +448,14 @@ public class Contacts.AvatarDialog : Dialog {
 	  }
 	}
 
+#if HAVE_CHEESE
+	/* Ensure the Vala garbage collector disposes of the Cheese widget.
+	 * This prevents the 'Device or resource busy' warnings, see:
+	 *   https://bugzilla.gnome.org/show_bug.cgi?id=700959
+	 */
+	cheese = null;
+#endif
+
 	this.destroy ();
       });