Base World, Body and Fixture on QObject instead of QQuickItem

This gives much more flexibility and less overhead.

It also gets rid of the automatic discovery of bodies to animate. However, this may not be as inconvenient as it seems.

Only 'demolition' example adjusted so far, for testing.

Not ready for merge. I'm quite convinced this is the right direction to take, but this pull request is mainly to get some feedback at this point.