Add skin layout customisation per ruleset

[Gonzalo-Bruna]

Describe the new feature:

Not sure if this is the same approach as #6124, since that opened issue wants to select a different skin for each ruleset, but this one aims to customize just one skin (with the skin editor) for each ruleset separately.

The idea is to save the skin editor configs in a per-ruleset way, since every ruleset is different and every ruleset uses different space in the gameplay field.

For example: (images taken from a guy on twitter)

[frenzibyte]

Pretty similar scope and that issue already mentions "skin settings", but I guess this can stay open since it's focusing on the "saving" part.

[peppy]

Converting this to an epic to track the increasingly important requirement of having per-ruleset layouts.

This could be used to solve issues like:

• Hit error bar overlaps with taiko playfield on default argon skin on 16:9 #21949
• Leaderboard overlaps part of taiko playfield #20422
• osu!mania legacy skins don't place elements in their correct (mode) default position #22362

It would both allow for applying per-ruleset defaults, and give users the option to further customise from those defaults. There should also likely be facilities in the editor to copy full layouts between rulesets (maybe not required if we have clipboard support).

[peppy]

Scratch that, I'll open a new issue to keep things tidy.