Quickplay: Update background image to match designs

[minetoblend]

Adds the new background based on @vatei's designs.
Note that the colors might be a tiny bit off, did so to be able to use the OverlayColourProvider.

This also replaces the background for all matchmaking test scenes via the new MatchmakingTestScene class.
exceptions are the Queue-Screen related test scenes & TestSceneMatchmakingScreen since it should create the background itself.

[peppy]

Is this how it's supposed to look? Note that this means it will no longer display the active beatmap. Not sure if that is the intention.

Also, your PR crashes in gameplay. I've committed a fix but still unsure about direction. We're just reusing an old generic background after all.

[minetoblend]

Is this how it's supposed to look? Note that this means it will no longer display the active beatmap. Not sure if that is the intention.

The background behind the matchmaking screenstack would have to go sooner or later but without removing it looks like what I was expecting right now. Was planning that as a follow-up task (WIP image below) alongside the other layout changes.

About showing the active beatmap background, I was under the assumption that if it was shown it would only really make sense in the time between the beatmap pick/reveal and gameplay start? In that case it could only show after the beatmap has finished downloading which could look quite awkward if you're the last one to finish the download, since the background would switch to the map bg and then immediately after the gameplay screen starts which also uses the beatmap bg (but probably slightly different).
Vatei's designs also make quite heavy use of the pink accent colours in some places so I'm kinda worried about the colours potentially clashing if the beatmap background was visible in all places.