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