Various fixes

[freezy]

Various changes from the last months that need to get in before I can test the other PRs. Namely:

• Don't write all mesh data into the scene when importing (d'oh!!)
• Fix NPE which was a side effect introduced with the gate lifter component.
• Slingshot coil arms can now be any game objects, not just primitives. They can also be rotated in any direction (#431 todo)
• Rubbers can now be scaled (as in: the entire drag point ensemble), which is pretty cool.
• Rename asset browser class
• We have a ball creation and destruction event that we can subscribe to.
• Some asset browser fixes I can't remember