FlxSubState API cleanup

- FlxState.setSubState() -> FlxState.openSubState()
- removed FlxSubState.close(), there is FlxState.closeSubState() already
- removed FlxSubState._parentState
- FlxSubState.initalized -> FlxSubState._created
FlxSubState: make internals of closeSubState() less confusing
FlxSubState: add _parentState and close() again
Merge pull request #885 from Gama11/FlxSubState2

FlxSubState API cleanup