Draft: postmarketos-ui-gnome*: Refactor a bit, add some apps to gnome desktop's pmb_recommends

This does some refactoring of the ui-gnome* packages, and adds a few useful apps to ui-gnome's pmb_recommends. While testing the "gnome" UI on v22.12, I found it was kinda useless out of the box... no browser or anything, but it did have a terminal app installed (yes, I managed just fine, but others might not 😄)

These commits should probably be squashed before merging (I can do that after review/approvals), I kept them separate to hopefully make reviewing a little easier.