WIP: Compatibility with newer versions of Qt (compiles with v5.12)

There are a couple of problems with newer versions of Qt. They are pretty severe as the project doesn't even compile any more. That in turn is a problem for Archlinux-based distributions like Manjaro as the packages of SleepyHead cannot be installed any more (see https://aur.archlinux.org/packages/sleepyhead-git/ and https://aur.archlinux.org/packages/sleepyhead/ for reference).

This merge request is my proposal for solving these problems, so that the project compiles with current versions of Qt.

I'm not very familiar with the project, so I haven't been able to test the changes in commits 37a5a937 and 324f303d.

In addition, I'm in doubt, if the form report.ui is still used. That's the main reason why this MR is marked as work in progress. I would like to clear that up before letting the MR getting merged. If it's not used any more, it should be removed. If it is still used, it would be nice if you could give me a pointer where I can find this form in the application, so I can test, if it still works.