[Feature Request] Move unlock conditions to engine rather than theme

[jack5github]

I've heard from people in the Discord server that the unlock system of StepMania is currently tied to themes, rather than the engine itself. I feel that this is taking away from a cool little thing that we could take advantage of for the community packs. I posted a visualisation of what I think the unlock progression could be, where each stage past the first has a requirement, and songs by the same artist will have requirements relating to the previous song. Unlock progressions can help solidify a theme for a community pack, similar to modern rhythm games like Arcaea and Spin Rhythm.

The requirements that we put on these songs should be very easy to accomplish so that you don't have to be good at the game to unlock every song, you just have to play a bunch. Harder requirements we can save for courses, see my post on courses.

In Super Smash Bros fashion, we could make it so that you only unlock one thing at a time if multiple things need to be unlocked. If that's the case, then based on the chart, the engine should prefer songs in the earliest stage and closest to the left.

Ultimately this is all your decision, but I would like to see comments being made on a bunch of the features around here. Don't want to talk to a brick wall, if you know what I mean.