Add Doomy's SteamTunes

[DoomyMcDoomface]

Ships Doomy's SteamTunes 1.5.0 from the production branch.

• Commit: 916297df99ae9855e7fbc057eba659c21a1f30d9
• The submodule tracks production. Further work stays on main and dev until the next release merge.

Millennium compares the installed commit with this pin. After this pull request is merged, Settings -> Updates shows the patch.

1.5.0

• Millennium lists this plugin as Doomy's SteamTunes.
• First install uses Steam Blue. A color already chosen in Appearance stays as it is.
• First install also turns on gapless playback with a 2 second crossfade, match-loudness at -20 LUFS, ducking at 80%, and a flat EQ. Spatial audio stays off. A setup already saved is left as it is.
• An empty library tells you no music was detected and opens Settings so you can scan a music folder.

[DoomyMcDoomface]

Apparently I will need to refactor this without AI logic on the audio processing (if I want to publish it). This started out as a quick repackage (pre 1.3.6) of the now-canceled Steam Music Player, and eventually branched off into other features such as live audio-ducking and onboard EQ. Probably not going to go through the trouble of refactoring post 1.3.6 because simply put: It's just a music player, it's not that big of a deal to me. But in the event I hand-code this down the line, I will reopen this PR.