Rewrite debian/rules:

- switch to cmake build system
  (after changes in upstream)
- enable all hardening flags
- delete get-orig-source section
  (after bumping of Standards-Version)
- delete override_dh_strip section
  (after deleting of libqxmpp0-dbg package)
- delete --parallel and --list-missing options from dh
  (after updating of debian/compat)