Schema validation to enable uploads to Beat Saver

[joshwcomeau]

A few days ago, Beat Saver added schema validation - this means it checks the .dat files to ensure that no superfluous properties are hung on it, outside of _customData.

Right now there are a few cases where I add custom data in "illegal" places:

• [x] "selected: true" on notes
• [x] "color: string" on bookmarks

There may be others, I'll do some testing

[joshwcomeau]

Was surprisingly annoying, but I got all of 'em =)

One known issue: if your map was created before today's release, and it features multiple difficulties, you'll need to make a change to every difficulty. You can simply add + remove a note for every difficulty. Then download it and upload.