UI: Allow any mod sound to be selected as a multiplayer turn notification sound

[SomeTroglodyte]

... Actually, "New architecture to find any non-texture-atlas media" would be more fitting. but that title up there can go into the changelog.

No issue, this is for a slightly miscast "Modding request": Allow any mod sound to be selected as a multiplayer turn notification sound. - borrowed the title even though my first one was a tad shorter.

This is the extreme version - started with "how would I imagine an easy to use yet still flexible tool to answer the question 'which media file if any should I use for a given key'..." and then coded top-down from the framework draft. I did not yet check whether the other intended clients do anything in a significantly different way. I suspect the priority between newgame-selected mods, perm audiovisual ones, and builtin is not always exactly the same - but it should be.

The ugliest part of that request is how to attach human-readable labels to sounds, the old solution to that was ugly, and it's not much better here. As mentioned in my replies to the above, moving UncivSound back to enum-based - nope, without large refactorings that ultimately fails on unexpected impediments. Refactoring all UncivSound usages to cleanly distinguish between the interface and the enum - possible, but not right now. Then once that enum exists, it could carry display labels... Again, not yet. But in here there's still a tricky little imp that still enumerates the existing UncivSound.X instances almost as if the container were an enum. Almost moot since most instances need a mapped label anyway, but - had fun coding it.

No screenshots since I don't know of any suitable mod

@5cover - your idea and you mentioned PR'ing the same - so your review would welcome, s'il vous plaît.

[5cover]

Sounds good. This solves my modding request and allows new sources for the notification sound :

1. sounds from permanent audiovisual mods
2. attack sounds of units from non-base rulesets (which was a workaround I attempted when 1. didn't work)

I'm not exactly familiar with how localization is done in Unciv - but does it work properly for builtin sound names and the "Attack Sound" suffix?