The logic in `LegacyBeatmapEncoder` that was supposed to handle
the lazer-exclusive feature of supporting multiple slider segment types
in a single slider was interfering rather badly with the Bezier
converter. Generally it was a bit difficult to follow, too.
The nice thing about `BezierConverter` is that it is *guaranteed* to
only output Bezier control points. In light of this, the same double-up-
-the-control-point logic that was supposed to make multiple slider
segment types backwards-compatible with stable can be placed in
the Bezier conversion logic, and be *much* more understandable, too.
Addresses one of the points in https://github.com/ppy/osu/issues/31496.
Not going to lie, this is mostly best-effort stuff (while the refetch is
happening, metadata lookups using the local source *will* fail), but I
see this as a marginal scenario anyways.
Reported at https://osu.ppy.sh/community/forums/topics/2015478?n=1.
Would you believe it that this button that has been there for literal
years never did anything?
Implemented at a per-beatmap level. Also additionally added to context
menu (at @peppy's suggestion), and also copy reworded from "Delete from
unplayed" to "Mark as played" because double negation hurt my tiny
brain.
A big part of these changes is refactoring, which is somewhat necessary
because it was previously implemented as two separate pathways which
in-fact need to be joined at the hip when handling spinners.
I've chosen to use `IHasLegacyHitObjectType` here because there's no
other flag that allows us to tell `ConvertHold` apart from
`ConvertSpinner`.
This also optimises the manager classes to better support `Live` usage
where the managed object is already in a good state (ie. doesn't require
re-fetching).