GUIBaseContext :: doAfterShow - Post-event handler after showing control

The GUI event is signaled, when a control is shown (GEV_Show event), which was hidden before. The handler allows e.g. for selecting an instance in the control.

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 post-events.
• 0 (ok) - Event handler terminated successfully.