[Feature] Split 2D boxes from generic covers across all media providers

[PodunkPedant]

Is your feature request related to a problem? Please describe.
With a new media category for 2d box fronts from Screenscraper (#4139), now there's inconsistency across media providers on whether a scraped image is considered a generic cover or a more specific box cover.

For example, I believe Launchbox, ES-DE, RetroAchievements, Libretro, Moby, and TheGamesDB can all be expected to serve traditional 2D box fronts, but RomM classifies them all as a generic cover. And since specific box2d images can only be provided by ScreenScraper right now, that means anyone who has already scraped or imported traditional box art as the cover will be left with duplicates the next time they scrape from ScreenScraper.

That's the situation I'm in. I've managed to import higher quality 2D box art from EmuMovies as the default cover for most of my games, but now those images are in a weird limbo in RomM's database. They're still served as the default cover when I select "2D Box" in the settings (which is also a misleading label now), but they're not listed as the 2D box in the game's artwork tab, and they're not used for the rotating 3D model either.

Describe the solution you'd like
RomM should clarify and enforce separate image categories for generic covers vs. 2D box art across all media providers.

• All sources that provide standard 2D box fronts — Launchbox, Libretro, potentially EmuMovies ([Feature] Emumovies artwork integration #1233) — should have those images recognized as box2d along with ScreenScraper
• All sources that provide generic poster art — IGDB, SteamGridDB, Launchbox sometimes (under Fanart - Box - Front) — should have those images recognized as a generic cover, or maybe renamed to poster for clarity
• "Boxart style" in settings should offer separate options for 2D box vs. poster (and probably 3D box image vs. 3D box model as well)
• Default cover / poster images shouldn't be handled differently from other image types. They should be listed in a game's Artwork tab alongside 2D boxes, 3D boxes, and the rest.

Describe alternatives you've considered
My suggested solution wouldn't actually fix my personal situation, with custom 2D box art saved to the cover category. The only way to fix that would be to migrate those images from cover to box2d, but there's no way to know who's in the same boat as me in order to run that migration automatically. I don't know how an optional or manual migration would work either, but I suspect either would be more trouble than it's worth.

Really, I think the better solution for that part is to add more options for scraping or importing media so users can fix those issues themselves. For example if EmuMovies was added as a media provider (#1233), then I could re-scrape all of my high quality box2d images from them and fix the problem that way. Or if I had a way to import a folder of locally saved images without using ES-DE or Launchbox as a go-between (#4257), I could fix it that way instead.