Fix display of string lists

Summary:
toString() will output an empty string when
it is applied to a string list with more than one entry.
Add an explicit path for string lists and do the correct
join()

Reviewers: ngraham, #baloo

Reviewed By: ngraham, #baloo

Subscribers: #baloo

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D17322