GUIBaseContext :: doBeforeDrag - Pre-event handler for Drag events

The pre-event handler for drag events (GEV_BeginDrag) is called before the drag action will be executed. The handler allows canceling the drag action by returning 1.

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.