Fix all imports, each class has its own file now, misc issues

I [imported all classes](https://gist.github.com/Gama11/5895453) (this might be useful for testing purposes!) to see whether they compile. There've been quite some issues actually, mainly because of imports.

There are still issues with:
- `FlxCoreUtils`
- `PxTextField.set_alpha()` on native targets
- `FlxKongregate` on native targets
- `SfxFader` because of edfa60db499b3abfdecefe9a34d44ffa806a366b I think
- the nape classes

Also, now each class also has its own file.