Keyboard input refactor

- moved keyboard related classes to new subpackage `system.input.keyboard`
- renamed `FlxMapObject` to `FlxKey`
- general cleanup
- Introduction of `FlxKey.RELEASED` etc for key states instead of using -1, 0, 1, 2
- Added `FlxG.keys.ANY`
- Inlined functions where it made sense (for all key properties in `FlxKeyboard` for example)
- Moved some vcr related vars from `FlxGame` to `VCRFrontEnd`
- `WatchEntry` and the vcr are now also using `FlxG.keys.enabled`, decreasing the lenght of the event listeners in `FlxKeyboard`

Probably forgot something. Tested this with a few demos and everything seems to be working, the vcr too.

Edit: Also added some docs here and there.