- Create new Spacecap logo icon.
- Add Linux tray support. Unfortunately we have to pull in GTK for this,
  because that's just what SDL3 relies on. As of right now it increases
  the AppImage build from ~18MB to ~26MB.
- Update build and test action to also build the AppImage.
- Update imguiz. This updates imgui to the latest docking branch. It
  also bundles SDL3 in with it, so we can remove the SDL3 dependency
  from Spacecap. I did this with these changes because we rely on some
  SDL3 APIs for this tray change.

closes #87