Bug #495: Activation Range

This is superseded by pull request #507

* Changed MWWorld::PhysicsSystem to report the correct distance for
hits.
* Changed MWWorld::World to remember the distances when using occlusion
queries.
* Changed MWWorld::World::getFacedHandle to return a pointer to last hit
object only if it is within activation range.
* Changed users of getFacedHandle to deal with the change in signature.
* Broke MWWorld::World::update into several functions.