Update the displayed BPM at song select with rate adjust mods

[peppy]

This only covers constant rate rate adjust mods. Mods like wind up/wind down will need a more complex implementation which we haven't really planned yet.

Addresses a portion of #10580.

20210225.172548.dotnet.mp4

[smoogipoo]

I've moved the bind of ruleset.ValueChanged back into BDL. This is still pretty incorrect simply because we're on an async context, but I feel safer this way for the time being to not inadvertently break anything.

Something to keep in mind when fixing this class is that every change of beatmap + enter of song select will cause two wedges to load - one via the synchronous execution of setting Beatmap, and another via the callback from the difficulty bindable.

No doubt this is contributing to some stutters/LCA overheads as this causes a beatmap conversion.

[smoogipoo]

I'm done with this branch, leaving the merge to you @peppy / @bdach

[peppy]

@smoogipoo are you saying that it was always loading two wedges?

[smoogipoo]

Yes, that's been the case since the dynamic SR stuff it seems.