Adds an event filter to this, allowing you to intercept events before they reach GDK.
This is a low-level operation and makes it easy to break GDK and/or GTK+, so you have to know what you're doing. Pass null for this to get all events for all windows, instead of events for a specific window.
If you are interested in X GenericEvents, bear in mind that
XGetEventData has been already called on the event, and
XFreeEventData must not be called within
data to pass to filter callback