Add first run setup dialog

[peppy]

I hope to add more screens to this, but for now I chose one to work with just to get the general structure setup. Quite a few lines of change, but also few modified lines (mostly test and new UI).

Eventually I'd like to convert this to a Screen which displays before the MainMenu but making that work (especially for the user-triggered display case) is a bit hard, so I figured getting the general feel and flow together is a good start. Making it a screen would allow improving the first-import dialog pop (which is a bit weird atm) and also simplify the notification blocking flow, which can best be described as a hack at the moment.

osu.2022-04-19.at.08.50.00.mp4

Follow-up screens will come in separate PRs. This is mostly about the overall flow (and the choice of implementing the UI scale screen first may have been a bad one, as making it work smoothly was the largest part of this implementation).

[bdach]

Skimmed this, nothing major found, left a few comments but they're mostly discretionary. I'm explicitly not leaving comments on the stuff that was pre-empted as temporary (i.e. the exposition of overlay activation state as public)

[peppy]

@arflyte can you take a look at the video in the opening post and let me know if you have any existing designs which use the new slanted language that i could draw from to update this to fit in?

[arflyte]

I haven't finalised any slanted designs for pop-ups so far. So I think it looks fine for what it is now.

But, if you want some idea, you can look into this file. (Warning, this is just an idea board, so it's ultra god-forbid messy >_<). https://www.figma.com/file/E6kgzLP6CGd62NPSRXLkUX/General-Overlay?node-id=0%3A1

[peppy]

I'm going to migrate this to use the newer design language, but the majority of the content should not change so this PR is still in a reviewable state. Will probably come as a separate PR.

[peppy]

I've finished updating the design of this to use the new shared logic. If no one has gotten too far in review yet, shall I push directly on this branch? I think it simplifies the code if anything.

Ended up splitting out as there was a bit of extra logic required.

[bdach]

This seems like a good start 👍 I've merged master here, resolving a trivial conflict along the way, and updated IScreen method signatures in line with framework changes. All changes look simple but please make sure to double check that I haven't broken anything.