Remove fast tabbing
The focus_chain is deprecated in GTK+3 (#8139 (closed)) but fast tabbing relies on it. Without focus_chain, it is not possible to implement fast tabbing without losing accessibility (allow focus on read-only widgets). (Or we must re-implement the focus_chain on all Containers).
I think we can safely remove the fast tabbing feature because it has become less and less important when we moved most of the buttons inside the entries.