More cleanup.
Remove the type ID system from SceneNode.

This feature was intended to make it easier for a client application to associate a type to a scene node. The problem is that this goes against the whole point of having a component system where there isn't really the idea of a "type" of scene nodes. Removing this just keeps everything that tiny bit simpler.

The serialized formated has changed with this commit.
Improve the way scene nodes are created in the scene editor.

This was supposed to have been done in the previous commits, but I forgot. :(
Cleanup.
Use the notion of "removing" scene nodes instead of "deleting" in the scene editor.
Fix a couple of physics related bugs.