RFC. Closes https://github.com/ppy/osu/issues/18169.
Implements the given proposal of keeping the current stable order but
adding a shuffle facility to the now playing overlay, and enabling it by
default.
There are more changes I want to make here but I'd like this to get
discussion first, because I am likely to continue putting this sort of
selection logic into `MusicController` and I just want to confirm nobody
is going to have a problem with that.
In particular this is not sharing the randomisation implementation with
beatmap carousel because it doesn't generalise nicely (song select cares
about the particular *beatmap difficulties* selected to rewind properly,
while the music controller only cares about picking a *beatmap set*).
One of my pending work items for post-realm merge.
The lowest-level import task is no longer asynchronous, as we don't want
it to span multiple threads to allow easier interaction with realm.
Removing the `Task` spec simplifies a heap of usages.
Individual usages should decide whether they want to run the import
asynchronously, by either using an alternative override or spooling up a
thread themselves.
