As FontAwesomeFX doesn't seem to be updated anymore and also doesn't include the latest 5.0 icons, we should switch to Ikonli. They have native JavaFX support as well.

I'll mark this as good-first-issue, because the code changes should be relative localized. However, there are changes required to many files, including grade build resources - so this task is perhaps a bit more complex than our average good first issues.