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