Create new elements in editor

[freezy]

Right now, the editor can load and save VPX files, but new elements will be ignored. So instead of looping through the imported data structure, we should loop through the game objects and pull the data from authoring components (the Behaviors which I'll rename to Authoring soon).

This has a few consequences:

• We should make sure only objects under the corresponding TableBehavior are exported, and if possible refuse to add those components outside of that tree.
• The idea is that the author adds empty GameObjects and then adds the relevant authoring component on it - or maybe we can create an editor toolbox that does those steps at once.
• When an authoring component is added, the relevant meshes should be created as well. It's desirable that children of a component cannot be moved around (e.g. the rubber and base children of a flipper)

[freezy]

Obsolete, item creation is covered by #147 and the rest in #149.