win32: Add a public clutter_win32_handle_event function

This function handles a single windows message. The idea is that it
could be used by clutter-gtk to forward on events from a
GdkEventFilter. The function replaces the old message_translate()
function. That function didn't translate the event anymore anyway and
instead it could generate multiple events so
clutter_win32_handle_event seems like a more appropriate name. The
function returns TRUE or FALSE depending on whether the event was
completely handled instead of setting call_window_proc.

doc/reference/clutter/clutter-sections.txt

@@ -1256,6 +1256,7 @@ clutter_win32_disable_event_retrieval
 clutter_win32_set_stage_foreign
 clutter_win32_get_stage_from_window
 clutter_win32_get_stage_window
+clutter_win32_handle_event
 </SECTION>
 
 <SECTION>