port to Qt6

Please merge !77 (merged) and !78 (merged) first.

Draft because:

• Phonon includes aren't working Fixed in !91 (merged)
• QNetworkInformation doesn't seem to work properly for me (no available backends) just had to install qt6-networkinformation-glib and/or qt6-networkinformation-nm
• The progress bar has some text that overlaps the actual bar Fixed in !93 (merged)
• Need to check JobTracker if some methods are still needed (they were changed upstream, should probably change here too)
• Need to check how to include Qt6 build of libktorrent in CI
• Group view crashes because at some point a QModelIndex's internalPointer isn't compatible with the expected type (Item *) Fixed in !82 (merged)