ibusCandidatePopup: Set OSK candidates list visibility correctly

Right now, we don't pay much attention to visibility hints from IBus
about the candidates list for the OSK suggestions bar. Since some
IMs rely on this visibility being honored, do that.

To fix this, the visibility hints for the lookup table are now
propagated to the keyboard, so the Suggestions actor hides its
internal candidate buttons. Since the Suggestions actor gets its
minimum height from CSS, this does not result in OSK size jumps.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5601
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2347>
(cherry picked from commit 237ba24d)