Refactor base difficulty and map objects

[XAce1337manX]

• Changed base classes of map objects to be concrete classes. It's the internal model CM uses for data. (e.g. BaseNote)
• Version specific classes are used only to parse from and to json.
  • Two methods: GetFromJson and ToJson.
  • Any supported currently custom json data keys
• Settings.Instance.MapVersion is a global setting for what version to output to
  • This is set only when loading a map and switching version
  • This is what node editor checks to get and parse json

Things left todo:

• Test this more
• Warn user on save if features used is incompatible with v2 (i.e. check for chains, arcs, angle offset, obstacles types, and rotation events outside v2 range)
• Bug: placing customEvents have default non-null values for duration, repeat, and worldPositionStays on placement instead of null