Rebrand: part 1
Remove the platforms we won't support, refactor everything. Move plugins into the main app. A few preparations for GTK4 as well.
This is really just a dumping ground for everything I've been working on when waiting for the repo to land, so it's all at once, except for GTK4. That one still needs a bit of work so will come after this is merged.
Edited by Alice Mikhaylenko