Move unnecessary properties from `BeatmapInfo` / realm to `IBeatmap`

[bdach]

RFC. PRing for early feedback.

• Closes https://github.com/ppy/osu/issues/20883
• Supersedes / closes https://github.com/ppy/osu/pull/23418
• [ ] Needs more testing probably, I've only checked that tests pass

This is basically https://github.com/ppy/osu/pull/23418 but hopefully better (because it moves out more stuff, and hopefully contains less naming discussions). This removes everything that was feasible to remove from BeatmapInfo, therefore bypassing the reference handling nightmare that causes https://github.com/ppy/osu/issues/20883.

I'm necroing that because it came up in recent discussions about persisting the new grid settings somewhere (see discord, cc @OliBomby).

Some things still remain:

• BeatmapInfo.BeatDivisor - can't really be moved out of realm because it's used for filtering in song select
• BeatmapInfo.EditorTimestamp could go live in the .osu file but I didn't want to make that change here as I would rather keep this diff strictly menial.

I'm not very happy about how much this makes IBeatmap grow but I don't see much wiggle room. Any effort to constrain interfaces any more to specific usages (diffcalc, editor) is basically killed by WorkingBeatmap / conversion and requires at least introduction of generic messes and multiple scoped overloads so I decided I'd rather not.

[bdach]

Generally I'll just bring up that while this is a draft I'd like to see movement on this or an alternative solution since the underlying issue is currently blocking editor improvements that users want to see too (like bookmarks which don't work because they get dropped by the dumb BeatmapInfo reference juggling logic).

[OliBomby]

I'll try to put in my 2 cents.

If the BeatmapInfo is for realm databased properties, I suggest renaming it to BeatmapDatabasedInfo or BeatmapRealmInfo. That will help convey its purpose and explain which properties you want in that class.

For the properties which should not be databased, I suggest putting them in classes like BeatmapGeneral for general properties and BeatmapEditor for editor properties. This mirrors the sections that exist in the .osu format and follows the precedent set by the BeatmapMetadata and BeatmapDifficulty classes.

[peppy]

Generally I'll just bring up that while this is a draft I'd like to see movement on this or an alternative solution since the underlying issue is currently blocking editor improvements that users want to see too (like bookmarks which don't work because they get dropped by the dumb BeatmapInfo reference juggling logic).

I have this on my review queue, if you need some assurance that it hasn't been forgotten.