SelectByGeometry with callback notification

[jng]

The current IMapViewer.SelectByGeometry API does not allow for one-shot notifications if the method call actually changes the selection.

The current (naive) ugly approach would be to subscribe to SelectionChanged, call the SelectByGeometry method and un-subscribe from the event if the handler is called, which is ugly and cumbersome.

Instead, we should have an overload of SelectByGeometry that allows for a System.Action to be passed in that is called when the internal selection has been modified, avoiding the need to do this event subscription mess