GUIBaseContext :: doBeforeDeleteData - Pre-event handler for DeleteEntry, DeleteSet and RemoveEntry action

The handler handles the DEV_Delete data event in a DeleteEntry, DeleteSet or RemoveEntry action. When the event is fired, an instance in the property handle is still available and can be accessed. The property handle for the instance going to be deleted can be provided by calling currentProperty().The event handler may prevent data from being deleted or removed by returning true.

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 (refuse) - Requested action will not be executed.
• 0 (ok) - Action is permitted to be executed.