Cleaning up the flatpak

[TheJackiMonster]

As mentioned in the PR from the flatpak here. The statement import PyQt5.QtWebEngineWidgets as webengine seens to be necessary in the code for the PDF preview to work in the flatpak. I'm not sure why this doesn't seem to be an issue in any other version but I assume it would be best to load this dependency either in our usual startup script at bin/manuskript before calling main() or adding it into the file responsible for the PDF preview.

Another aspect I promised to do for the next release is getting rid of the .appdata.xml-file and the .desktop-file in the flatpak repository. Manuskript should only have a single .desktop-file and a single .appdata.xml-file in its repository after cleaning it up and the flatpak will reuse them instead of maintaining own files separate.

[TheJackiMonster]

I've already done the necessary part of this in this repository. The commit for the flatpak is ready to be published. Therefore I will close this already.