Fix file path intercompatibility for Windows and POSIX systems

[searinminecraft]

Fixes #298

This fixes a bug where if you move a library saved on Windows to Linux or any POSIX operating system, it will cause the files to not be found due to backslashes. This is a hack I done to prevent this from happening.

Needs extensive testing though, even though that I have confirmed this fix works and does not crash Tag Studio.

cc @coolesding

[searinminecraft]

Well thats a concern.. putting Path() on it would just make the bug return. (it becomes \ again, making the as_posix() useless). Is there any other thing i can do?

[CyanVoxel]

Since Windows-style backslashes are valid characters in Posix paths, it makes things a bit tricky.

I would say the best course of action to fix #298 would be to always save and load file paths inside of ts_library.json as Posix paths moving forward, regardless of the OS. In addition there would need to be consideration for libraries saved with versions before this fix (currently 9.3.2 and earlier) - I propose replacing any "" characters with "/" in older library saves. This would, however, break data inside of anyone's libraries on Posix systems that currently contain file entries that use backslashes as legitimate file characters. I believe this can be avoided by performing a "file exists" check on both the old and new file paths, and only performing the conversion in cases where the swapped path comes back valid. If both cases come back false, then there's an existing unlinked entry that can't be broken any further from us messing with it.

[searinminecraft]

I have no motivation in trying to fix this. Sorry...