accessibility support for GTK 4

One of the last missing pieces for GTK 4 support is around accessibility. Primarily that was held up because GTK 4 didn't yet have a GtkAccessibleText implementation.

GTK 4.14 will have that, and we should make VTE support it. I think that is somewhat a requirement for a GTK 4 based gnome-terminal as well.

I tried to keep the design of the GTK 3 version although in my opinion, considerably cleaned up. There are a few crufty bits around integration which I'd love to have a discussion about how we'd want to do it to be the most maintainable.

Initial patches incoming...