Adapt for use in GNOME

After comparing gnome-initial-setup to carbon-setup, and weighing carbonOS's needs against the possibility of using an upstream utility (though not fully, we'd need to fork it), I've decided to make carbon-setup a drop-in replacement for g-i-s.

This MR implements that.

Part of build-meta#118 (closed)

Things left to fix:

• ibus not running
• try mode doesn't start
• Make the installer icon show up in the liveos session, and pin it to the dash
• Make a11y menu show up in initial-setup session
• Install -> Select a disc -> Confirm dialog doesn't have a parent
• Installation failed dialog has no parent
• Fails to copy settings over w/ permission denied errors
  • Isn't copy-helper running as root? Maybe not
  • Potential fix: make it run as root
  • Potential fix: drop new user's UID into /run/gnome-initial-setup/gnome-initial-setup-uid, let GDM chown the dir, then have a user unit copy the files on session startup
• Need a more specialized mode from gnome-shell
  • disable screenshots
  • disable settings
  • disable lock
  • disable battery settings (?)
  • disable logout
  • disable sleep
  • DON'T disable shutdown/reboot
  • should apply to greeter as well: disable dark mode toggle, stay awake toggle
• Initial setup window should be centered