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.
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