Issue with library directory operations

[ronso0]

This is a meta issue collecting some issues with regular (expectable) directory operations as well as cross-OS library migration (I tested Windows <--> Linux, currently neither direction works).

1. Can not remove missing directory (e.g. unmounted drives)
   Fixed by (fix) relink directory / migrate Linux/macOS <--> Windows #12878

2. Can not add a new directory if one of the current directories is missing because it is tested whether the new dir is a child of one the current dirs.
   -> this is hard to fix (or rather it's easy to fix by reverting from mixxx::FileInfo back to QString, but that is obviously suboptimal)
   Well, fixed by (fix) Library: allow adding new dirs while we have missing watched dir(s) #12937

3. Relinking music directories set on a Linux machine to Windows paths or vice versa is not working, it just fails silently, see Relink music Directory from Linux to Windows fails #12715 (probably also the other way around)
   Fixed by (fix) relink directory / migrate Linux/macOS <--> Windows #12878

4. Starting on Linux with Windows music paths causes debug assertion hits (presumably during cover art lookup, relevant only for debug builds

[ronso0]

Closing this now that 1 -3 have been fixed.