const, pass by ref, and virtual normalizing

This is a big pull request. But almost all of it is one line or one statement changes.
80% of this pull is as follows:
function() +const;
-const val function();

However, while adding these changes to physics_actor.i, I realized a not-insignificant amount of virtual functions weren't properly labeled or used! The entirely wrong function would be called in many cases, and things meant to override functionality wouldn't be called at all, notably in PhysicsActor. This lead to an effort to correct these, which is why this pull is so large; there is no use in correcting the interfaces twice, so I do it once.

The only breaking change is in TextActor; it now returns a BoundingBox by value, to match with Actor. As an rvalue may be bound to a constant reference, this change is actually probably not breaking.