Fix `xdg-mime` default registration

[jaimergp]

Description

Closes #226. Instead of using xdg-mime default, we edit mimeapps.list manually. If an application is already registered as default, we won't override it. We always populate the "Added associations" list with our desktop file so it's there in the "open with" menu. We also handle cleanups now.

Checklist - did you ...

• [x] Add a file to the news directory (using the template) for the next release's release notes?
• [ ] Add / update necessary tests?
• [ ] Add / update outdated documentation?