Give the awake property of Body a change signal

[mitchcurtis]

This would be useful for animating an idle state of a game object, for example. I don't think that there's currently any way to be notified (via QML bindings) that a body has stopped moving.

[bjorn]

Yep, there isn't, because I wasn't sure whether it would be useful while at the same time adding it would require checking the awake state for all bodies after each step. But the performance impact should be minimal so it could just be added.