Backport 5.9/Master's GroupDialog code to 5.8.
Summary:
This brings us to a common baseline on the three active branches
and addresses regressions on the 5.8 branch.
5.9's code added a scrollbar and improved keyboard nav.
BUG:378042
Reviewers: Plasma, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D5287