Add isJuggling() to Juggler and getTouchByID to TouchEvent

1. I have a static constant that needs to be added to Starling's juggler. If the Starling instance is disposed, and a new one is created, then this will help me determine if I need to add my constant to the new juggler or not.

2. We discussed getTouchByID() previously. This will help avoid a lot of redundant multi-touch code where one only cares about a specific touch ID after TouchPhase.BEGAN.