GUIBaseContext :: doBeforeClose - Pre-event handler before closing control

The handler handles the GEV_Close GUI event when closing a control. When the event is fired, the property handle is still opened and data can be accessed.

The handler should return one of the following return codes:

• -1 (not handled) - the event handler will be removed from the handler list and is not called anymore.
• 1 (error) - Returning an error has no effect for this events.
• 0 (ok) - Event handler terminated successfully.