We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

This was a noop in all of our apprts and I think is a holdover from the
glfw days.

Ghostty has had support for a while (since PR #3124) for parsing progress
reports but never did anything with them. This PR adds the core
infrastructure and an implementation for GTK.

On GTK, the progress bar will show up as a thin bar along the top of
the terminal. Under normal circumstances it will use whatever you have
set as your accent color. If the progam sending the progress report
indicates an error, it will change to a reddish color.