Previously backspace would only ever remove a single character left of
the cursor, regardless of selection.
This requires the application to correctly set the anchor position in
text_input::set_surrounding_text(), which currently only gtk4 seems to
do. When there is no selection or on other applications that always set
cursor = anchor, like gtk3 does, the behavior is not changed and still
only deletes one character.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2746>
7a409bfffc