Hit object refactoring.
- Moved the bulk of hit object parsing into the OsuHitObject constructor, and made all fields private.  Only combo-related data is still set by OsuParser.
- Added 'isCircle()', 'isSlider()', 'isSpinner()', and 'isNewCombo()' methods for convenience.

Other changes:
- Fixed difficulty overrides are no longer affected by game mods.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>