Make Categories and Cross-posting independently toggleable #40
Description
Right now Categories and Cross-posting in the compose window are mutually exclusive — toggling one hides the other. Visibility also resets every time you open a new compose window.
I've been wanting these to work independently for a while, and I am taking a shot at implementing it. The panels now have their own collection views so both can be shown at once, and visibility persists via UserDefaults. I also think I can address #19.
I understand this might not be a change you want to take, but I'd be happy to open a PR for your review if you're interested.