This is already done in gdk_event_source_get_filter_window(), and
could lead to wrong event assignment if an event translator happens
to return a window for an event it doesn't handle.
This method can be implemented by event translators so they
return the right window from XGenericEventCookie events, as
ev->xany.window isn't meaningful for these.
GdkEventSource now also uses this to find out the right window
filters to apply.
The X11 backend exports a number of symbols which are _-prefixed
(so don't become part of the gdk api), but are not named in a
way to prevent accidental clashes between backends.
The one API change here is that the gdk_xid_table functions
have been removed - they did not server an purpose, since the
xid table only stores windows anyway, and we already have a
lookup-by-xid function for windows.
