some refinements - HUD works with arbitrary lines now, race is setup in physics setup
wip
some fixes for an earlier commit
added world-object-hierarchy with on_object_added-event that's propagated upwards.
renamed physics_setup with setup_within_world event
replaced world object list within world with a single root node.

now all object-addition is generating a callback event inside World::on_object_added
now the matrix for child-objects is preserved between renderings
race now relies on the car being set-up as part of the object tree
Merge branch 'enhance-world-object-tree'

WorldObjects now have a common WorldRoot at the top, hosted by World
initial file-renaming
renamed World to SceneGraph and WorldObject to SceneNode
some minor re-namings
Merge branch 'world-becomes-scene'

World is now SceneGraph, and WorldObjects became SceneNodes