caribou is dropped - the onscreen keyboard is provided by
gnome-shell now with 3.28.
Add cantarell minimum dep ensuring, as gnome-shell uses it,
and here we can enforce the minimum version meant by
upstream gnome releng.
Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>