Simplify GUI Proxy implementation & customisation.
- the proxy private is always the IGUIObject* pointer
- the private data is accessed through a safer wrapper
- CreateJSObject returns an opaque type to allow easier extension & prevent errors.
- The implementation of CreateJSObject is moved near the GUI Proxy template instantiation, and both are wrapped in a convenient macro (this makes it so that if you use the macro, you can't forget to overload the method, and vice-versa).
- The common IGUIObject JS interface no longer needs to be repeated.
- All specialisations are again put in the same file, for improved compile-time & clarity, given there are so few lines now.
- While at it, implement toSource which makes it possible to uneval components (does the same as toString though).
Differential Revision: https://code.wildfiregames.com/D3826