More serialization work. Converter selection issues.
Got character serialization working.

- Added support for level serialization (using LevelConverter).
- Fixed issues with gear serialization (using ItemConverter).
- Added equality and ToString() implementations to Level and subclasses.
- Removed LevelJsonData and subclasses.
- Removed ItemJsonData and subclasses.
- Omits the "slot" property on items when it is Slot.None.