Fix warning when reading plugin metadata

Summary: X-Purpose-PluginTypes is a StringList, not a String. Use an appropriate way of reading it.

BUGS: 408834

Test Plan: Share file in Dolphin. Don't get org.kde.kcoreaddons: Expected JSON property "X-Purpose-PluginTypes" to be a single string. but it is a stringlist anymore.

Reviewers: apol, aacid

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D21967