GUIBaseContext :: doBeforeContextMenu - Pre-event handler before creating context menu

The handler handles the GEV_ContextMenu GUI event when the application is going to display a context menu. Overloading the handler allows updating the context menu before being displayed. The event handler does not support suppressing the context menu.

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.