Fix taiko beatmap scroll speed increasing after every save in editor

[bdach]

Closes #15435.

There were two root causes for this error:

• The logic in BeatmapManager.Save() is tricky. Directionality of Copy{From,To} matters there, because the post-conversion editor beatmap is aware of the taiko adjustment, but the database model - a plain BeatmapDifficulty - is not. Using the wrong direction of BeatmapDifficulty.CopyFrom() rather than TaikoMultiplierAppliedDifficulty.CopyTo() caused the post-multiplied value to be stored to DB, which was then multiplied again on next conversion, causing the stacking effect.
• Secondly, there was a basic logic error in the CopyTo() implementation - the incorrect object was receiving the de-application of the taiko multiplier.

I'm well aware the fix is kinda shoddy, but let's be honest, so is the entire conversion process as we have it what with the haphazard cloning. The least good that can come of this is a temporary fix and added test coverage, which feels like a good-enough thing for now until that's addressed properly.

I also had to expose TaikoMultiplierAppliedDifficulty as internal for the test. Hopefully that's okay.

[peppy]

Nice one. Makes sense and is also painful to look at the solution.