add bookmark support

[spookyGh0st]

Just want to let you know, that bookmark support would be really great, since that lets other tools build on top of your mapper.

[StefanH-AT]

I will look into that after I implemented the basic features.

[StefanH-AT]

Can you clarify how this bookmark support should be implemented? Where does it appear in the json model and what is its structure?

[spookyGh0st]

It appears in the Difficulty file with a structure like this:

{ "_BPMChanges": [], "_bookmarks": [ { "_name": "sample bookmark at beat 4", "_time": 4.0 }, { "_name": "sample bookmark at beat 20", "_time": 20.0 } ], "_events": [], "_notes": [], "_obstacles": [], "_version": "2.0.0" }