Pixels per meter update

Here are three unrelated commits I'm putting together in this pull request. The real important one is the pixelsPerMeter update.

The use-case I have is a top-down sport game where the arena's meter size is proportional to the number of players, and this requires pixelsPerMeter to behave properly when updated in the middle of a scene.
If you make the `pixelsPerMeterChanged` connection solely based on whether a world is set, then I think you need to check here whether `mBody` is set. Though the only reason it may not be is when the signal gets emitted while the component is not ready yet. I'm actually not sure if this can really happen.