Some theoretical mp3tunes fixes + other tiny quality fixes

Fix the regexes so it compiles again. Add fixes to reported bugs, including a patch posted by Zhouyang on bugs.kde.org. Fix some issues reported by code quality scans.

The compilation works now, but no effort was made even to try to test the functionality, which probably doesn't. The service as is would need work and everything is turned off in CMakeLists. Hard to predict if someone will ever update the mp3tunes to usable state, but I guess there's no point in NOT applying the potential fixes (as the service still exists, so maybe not everything's not broken enough to justify removing) so the existing issues can be happily cleaned up at least.

Additionally some other tiny quality fixes

BUG: 419132

BUG: 391146