Various improvements

This series does a lots of changes in the packaging: removes old cruft/stuff, and fixes & updates existing bits.

Please refer to the changelog entry added with each commit for more details. I can improve/reword the commit logs and/or the changelog entries in case they need more explanation/reasoning.

There are still few minor things to improve, but I'd say that now the packaging is way simpler and more up to the standards of other KF5 applications.

BTW: if it can be helpful, I can volunteer to help with the maintenance of this package, so in case feel free to add me as member of this repository, so I can push commits and tags (and do uploads).