Implement a new UI settings screen

[NABN00B]

Fixes #16877, I think.
Hopefully improves #21223. @Domiiniik

• Moves all of the UI settings (except Language) into a new "UI settings" screen.
• UI and achievement sound settings are made a copy of.
• Added custom file picker for each UI sound.
• Moved the definition of AudioFileChooser from RetroAchievementScreens.h / .cpp to MiscView.h / .cpp
• Current tab is now stored for RetroAchievementsScreen (and also the new UISettingsScreen).

This isn't fully fleshed out for a release version, but I think it's good enough to be merged.
I've left a 'todo' in UISettingsScreen.cpp.

I heavily recommend renaming the string for the setting:
Transparent UI background
to:
Transparent pause menu background
because currently it's very vague / confusing.

Screenshots:

Tree of settings:

• General UI
  • General UI settings
    • UI size adjustment (DPI)
      • App switching mode [iOS]
      • Hide status bar [iOS]
      • Hide navigation bar [Android]
    • Mid-game UI settings
      • Transparent UI background
        • Show indicator when saving/loading
        • Notification screen position
        • RetroAchievements notification [links to RetroAchievements -> Customize tab]
• UI sound
  • UI volume
    • UI sound [checkbox]
      • UI volume
      • Achievement sound volume
      • Game preview volume
    • Customize sound effects
      • Select
        • Confirm
        • Back
        • Toggle on
        • Toggle off
        • Achievement unlocked
        • Leaderboard score submission
• Customize GUI
  • Background
    • Set UI background... / Clear UI background
      • UI background animation
    • Theme
      • Theme
        • Color tint
        • Color saturation
• Accessibility [empty]

AI disclaimer:
AI was used to generate the skeleton of the new UISettingsScreen class based on DeveloperSettingsScreen, to move the AudioFileChooser class and to help with includes (missing / redundant). No new logic was invented.

[hrydgard]

This seems like a good step in the right direction. I'll look at this again later, considering merging.

Unrelated: I'm wondering if we should think about supporting 3- or N-level hierarchy in the settings UI directly somehow.. it's kinda strange to go from 2 levels to the next 2 levels. Although I'm not really sure what that would look at. Maybe should look at the settings hierarchy for Android settings for example, problem is that they don't have a sidebar tab thing... Well. For later.

[NABN00B]

UI and achievement sound settings are made a copy of.

I don't know whether we should remove those from the original Audio tab and add a link to the new "UI settings ->UI sound tab" instead. (I definitely don't want to add all the custom SFX selection to the original Audio tab, way too much baggage.)