Add QML module built against Qt6 + general packaging cleanups
- Add QML module built against Qt6.
- Replace transitional pkg-config build dependency with real pkgconf.
- Build with hardening=+all build hardening flag.
- Drop calling dh_missing with --fail-missing, it’s been the default since
debhelper 13 compatibility level. - Tighten selection of files installed in the Qt5 QML module package.