Port to GTK 4.0

GTK 4.0 got released some days ago: https://blog.gtk.org/2020/12/16/gtk-4-0/

There are some cool new things in there that are all mentioned in the blog post. However, I marked this issues as blocked for now because I don't want to transition to GTK 4.0 before Debian stable ships it. I think we have good chances to have it in the next Debian release coming in 2021. If we focused only on Flatpak, we could transition to GTK 4.0 today, though.

Related issues: #80 and all the issues linked in there.