Clutter.Stage.get_key_focus() now always returns the value of
the Clutter.Stage.keyFocus property, whereas previously it
would return the stage itself when no explicit focus was set.
Adjust the code accordingly.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/561
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/395>
(cherry picked from commit e06b1e8236)