ClutterEvent: add API to query the full keyboard state when the event was generated

When talking to other applications or serializing the modifier
state (and in particular when implementing a wayland compositor),
the effective modifier state alone is not sufficient, one needs
to know the base, latched and locked modifiers.

Previously one could do with backend specific functionality
such as clutter_device_manager_evdev_get_xkb_state(), but the
problem is that the internal data structures are updated as
soon as the events are fetched from the upstream source, but
the events are reported to the application some time later,
and thus the two can get out of sync.
This way, on the other hand, the information is cached in the
event, and provided to the application with the value that
was current when the event was generated.

https://bugzilla.gnome.org/show_bug.cgi?id=706494

clutter/clutter-event-private.h

@@ -19,6 +19,13 @@ void            _clutter_event_set_platform_data        (ClutterEvent       *eve
                                                          gpointer            data);
 gpointer        _clutter_event_get_platform_data        (const ClutterEvent *event);
 
+void            _clutter_event_set_state_full           (ClutterEvent        *event,
+							 ClutterModifierType  button_state,
+							 ClutterModifierType  base_state,
+							 ClutterModifierType  latched_state,
+							 ClutterModifierType  locked_state,
+							 ClutterModifierType  effective_state);
+
 void            _clutter_event_push                     (const ClutterEvent *event,
                                                          gboolean            do_copy);