Samplers, crates, playlists: fix storing import/export paths

[ronso0]

Fixing a regression introduced by #4531 and #4539
on Windows the import/export path is not stored (affects import/export of sampler banks, crates and playlists)
https://bugs.launchpad.net/bugs/1964508

https://github.com/mixxxdj/mixxx/pull/4531/files#r829445200

I suspect this always truncates at position 0, see https://bugs.launchpad.net/bugs/1964508

Native separator is \ on Windows and maybe Qt has already converted the path to use /? In that case lastIndexOf("\") would return -1 which is equivalent to passing 0 to truncate
https://doc.qt.io/qt-5/qstring.html#truncate
It is not recommended to use QDir::separator() for constructing paths, but appearantly it also creates issues when computing paths
https://doc.qt.io/qt-5/qdir.html#separator

[ronso0]

Native separator is \ on Windows and maybe Qt has already converted the path to use /? In that case lastIndexOf("") would return -1 which is equivalent to passing 0 to truncate
https://doc.qt.io/qt-5/qstring.html#truncate
It is not recommended to use QDir::separator() for constructing paths, but appearantly it also creates issues when computing paths
https://doc.qt.io/qt-5/qdir.html#separator

Jep, confirmed on Win10
I'll prepare a fix

[daschuer]

LGTM, Thank you for digging it down remote.