VMManager/QT/FSUI: Basic .m3u playlist support

[nfriedly]

Description of Changes

This gives PCSX2 the ability to parse .m3u files for multidisc games. It automatically loads the first disc, and lists all discs in the System -> Change Disc menu as well as in a new FullscreenUI submenu that Change Disc opens when the current disc was launched from an .m3u file. (When not launched from a .m3u file, the FSUI Change Disc option opens the file browser as before.)

I'm not happy with how the new FSUI menu cuts off the file names, so I might have one more commit to widen it if I can figure out how.

Rationale behind Changes

I want this for Batocera, where .m3u files are the correct way to group multidisc games.

Fixes #6696

Suggested Testing Steps

I tested a Windows build, both via the System -> Start File menu in PCSX2, and by right-clicking a .m3u file in explorer and choosing Open with, then finding the executable for my modified pcsx2 build.

I tested the second commit by switching discs a few times.

I have not yet tested it in Batocera, but I think it will work.

I have also not yet tested how it handles save states where your on a subsequent disc. I'm moderately concerned that the "just load the first disk" behavior I chose could cause trouble there. (I don't have any save files beyond the first disk of any multidisc games.)

Did you use AI to help find, test, or implement this issue or feature?

Yes, I had copilot plan this out, then I tweaked the plan a bit, then had it implement it. I reviewed the code and made a few changes.

Future work

I think these ideas would make sense, but I'm not not planning on adding them in this PR:

• Remember which file of the playlist was last loaded, at least when automatically loading savestates
• Maybe make a hotkey to switch to the next disk(?)
• Maybe add .m3u support to the main "all games" list(?)

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[Deozaan]

I have also not yet tested how it handles save states where your on a subsequent disc. I'm moderately concerned that the "just load the first disk" behavior I chose could cause trouble there. (I don't have any save files beyond the first disk of any multidisc games.)

I did a search for online saves and found some that might work, such as this one for Xenosaga 3: https://hippyj3.blogspot.com/2019/09/pcsx2-full-save-collection-xenosaga.html

I'm sure other files are available on that site or other sites for disc-2 saves for various games that could be used for testing.

(I am not affiliated with that site. I have never heard of it until today. It was among the top search results when I searched for save files for PS2 games that had multiple discs.)