Support building against Qt6 + KF6
- Switch to versionless references where possible
- Use QT_MAJOR_VERSION where necessary
- Use KDE_INSTALL_SYSTEMDUSERUNITDIR if available
- Migrate away from QDateTime::{from,to}Time_t
- Add Qt6 + KF6 to CI
Only change in minimum requirements is the bump to Qt 5.15. Could be made Qt6/KF6 only, but with minimal benefits so we can just keep Qt 5 support around for a bit longer.
Draft because the Qt 6 version builds, but is otherwise untested. The CI can't run the testsuite