evdev: Allow to retrieve the input.h event code from ClutterEvents

This is now stored as platform data in the ClutterEvent, so can
be retrieved with the clutter_evdev_event_get_event_code() call
that's been added to the evdev backend.

https://bugzilla.gnome.org/show_bug.cgi?id=758238

clutter/evdev/clutter-evdev.h

@@ -131,6 +131,9 @@ void clutter_evdev_warp_pointer (ClutterInputDevice   *pointer_device,
                                  int                   x,
                                  int                   y);
 
+CLUTTER_AVAILABLE_IN_1_26
+guint32 clutter_evdev_event_get_event_code (const ClutterEvent *event);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_EVDEV_H__ */