GUIBaseContext :: doBeforeOpen - Pre-event handler before opening control

The handler handles the GEV_Open GUI event. The event is fired before opening a control. When the event is fired, the control and its subordinated elements are not yet created and the data source is not opened.

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 (refuse) - Returning an error prevents the control from being opened.
• 0 (ok) - Event handler terminated successfully.