memcard: Detect and attempt to repair broken index files from the v1.7.2115 bug

[xTVaser]

Description of Changes

The current folder memory card implementation appears to still be working as intended after the aforementioned fix #5356 but some people have been obviously caught by it. The folder memory card code is currently being re-written, so it doesn't make sense to go too crazy with re-writing.

If the index file is detected as malformed, it backs it up and re-creates it by scanning the directory. For most games, (i've tested with a few), this brings things back to a working state with no data loss.

Rationale behind Changes

Only the index file is broken because of the aforementioned bug, which only matters for very specific games that depend on the order of the files (ie. GTA).

Suggested Testing Steps

• Do not merge this too fast....let's let it sit for a while and be tested by people that ran into the problem.
• Intentionally break your _pcsx2_index file, you should see an error in the console when booting games and it should patch things up -- see if you can load your save files as expected
• Do significant folder memory card testing as well, load and save multiple times. Last thing I'd want is to create another bug trying to remedy an old one....
• Involve unicode in your memory card path and in the file names that are restored