GUIBaseContext :: doBeforeSelect - Pre-event handler before selecting data in a control

The handler is a reaction on the GEV_Select GUI event, which is fired, before a tab in a tab control is going to be activated. The event is not fired when selecting items in a list or in a drop combo box. When the event is called, the display level is the tab level going to be left.

The handler should return one of the following values:

• -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 event.
• 0 (ok) - Event handler terminated successfully.