Freeplay rework

[x8c8r]

Expansion of #156 and refactors Freeplay even more:

• Freeplay tabs are now required for Freeplay to function. Having no tabs defined will yield a warning like it used to when there were no weeks.
• Freeplay tabs can now be generated using pre-existing weeks like this:
  • Songs can still be appended using a songs array like before.

 {
  "title": "Friday Night Funkin'",
	"fromWeeks": ["tutorial", "week1", "week2", "week3", "week4", "week5", "week6", "week7", "weekend1"]
}

• Weeks no longer have any role in the song's appearance. Now all data about the song like the display name, icon and background color are taken from the song's meta.json file.
• Difficulties are also revamped, and are now picked per-song with these priorities:
  • The difficulties defined in the song's meta.json
  • Difficulties defined in the week file (if the song was part of a week)
    • As part of the rework, the difficulties in the week file are now also arrays instead of comma-seperated strings:
      • "difficulties": ["Easy", "Medium", "Hard"]
      • All parts affected by this change (Story Mode menu, Week Editor) have been updated as well and work properly.
  • Default "Easy", "Normal" and "Hard" difficulties.