M3U files required by cd-based beetle cores but not supported well at all by UI and features available for CUE scans

Description

Multiple problems with M3U files in the user interface as of version 19.0 (and as long as beetle cores have required m3u for multi-disc on-the-fly changing). M3U files are a requirement for using beetle cores that are for CD games with multi-disc support. Here's the problems that need to be addressed as it's very unintuitive to use M3U's but still required:

• Scanning directory by default does not scan in .m3u files, this has to be done manually. This doesn't make sense if it's required to use them for basic functionality.
• .m3u files are not checked for in the game information database (e.g. Chrono cross cue files get thumbnails and database information, m3u gets no thumbnails or information, just the title)
• If a game has an .m3u file being utilized (meaning it's a multi-disc game since this is necessary), the scanner should ignore the cue files of the same name/database entry, but currently only gets cue files, and as said before ignores .m3u. It ought to include only .m3u files for games that have them, and cue files for games that don't have an .m3u, to not import lots of unnecessary games. manual scanning does not help with this problem as you will get either only .m3u (which most games don't require), or both cue and m3u, or you exclude m3u as your only 3 options. That or you manually edit the playlist file laboriously if you have a large collection.

Expected behavior

I expect to "scan directory" the PSX directory and have my .m3u supported games have a thumbnail, game information from teh database, and my cue-only games to be scanned in like before, with the cue's for m3u-supported games being excluded from the list.

Actual behavior

Scan directory adds only cue files from the psx directory, ignores m3u, and if you manual scan the directory for .m3u, then it still doesn't use the games database and therefore gets no game information or thumbnails for the m3u entries. If you scan manually for m3u only, you are excluding cue, if you scan manually for cue, you miss m3u, and if you scan manually for both, you have excess cue entries from .m3u enabled games.

Steps to reproduce the bug

Import --> scan directory --> a psx folder with a mixed set of m3u-using games and cue-using games --> look at the psx playlist and see no m3u files --> manual scan for m3u --> update thumbnails --> m3u games are now added, but no game information and no thumbnails and redundant cue files for each disc have to be manually removed

Version/Commit

You can find this information under Information/System Information

• RetroArch: v1.9.0

Environment information

• OS: Windows 10 Home Feature Update 2004

Reproduce the issue through Import → scan directory on a mixed PSX folder, then compare directory scanning, manual M3U scanning, game-database matching, and thumbnail updates. Done means M3U games receive database information and thumbnails, cue-only games remain included, and cue files for M3U-backed games are excluded without manual playlist editing.