events: Add platform-data to allocated Events

Events allocated by Clutter should have a pointer to platform-specific
data; this would allow backends to add separate structures for holding
ancillary data, whilst retaining the ClutterEvent structure for use on
the stack.

In theory, for Clutter 2.x we might just want to drop Event and use an
opaque structure, or a typed data structure inheriting from
GTypeInstance instead.

clutter/clutter-backend.h

@@ -85,6 +85,12 @@ struct _ClutterBackendClass
                                                 ClutterStage    *stage);
   ClutterDeviceManager *(* get_device_manager) (ClutterBackend  *backend);
 
+  void                  (* copy_event_data)    (ClutterBackend  *backend,
+                                                ClutterEvent    *src,
+                                                ClutterEvent    *dest);
+  void                  (* free_event_data)    (ClutterBackend  *backend,
+                                                ClutterEvent    *event);
+
   /* signals */
   void (* resolution_changed) (ClutterBackend *backend);
   void (* font_changed)       (ClutterBackend *backend);