Add library built against Qt6 + general packaging cleanups
- Bump minimum required Qt5 version to 5.10 as declared by the upstream build system.
- Tighten selection of files installed into the shared library package to only match the current soname version.
- Build with hardening=+all build hardening flag.
- Replace transitional pkg-config build dependency with pkgconf.
- Drop explicit --fail-missing option for dh_missing, it has been the default since dh 13 compat level.
- Bump Standards-Version to 4.7.0, no change required.
- Also build the qt6 version of the library.
- Add symbols files for shared libraries.