Add Bounds support

There was a serializer for the Bounds type already. However, Bounds were
explicitly filtered out for some reason.
I have no idea why, because there were no comments. I assume that the
original developer forgot to remove filtering them out after writing the
serializer, because it seems to be working perfectly.
Extend BoxCollider support

Store the PhysicsMaterial of the collider
Extend CapsuleCollider support

Serialize PhysicsMaterial and direction
Extend SphereCollider support

Save PhysicsMaterial
Add MeshCollider support

Warning:
isTrigger and convex behave a bit weird at times. The developer needs to
make sure, that no problems exist in the scene prior to saving. Problems
will be saved AND restored!