use KPluginMetaData::readStringList()

Summary:
replace the last remaining place in pluginloader
where a string list was read as a normal string

Change-Id: I66e7f563745466e31cfb23492fdb081806b1e406

Test Plan:
related to D3902
dropping of files on containment still works

Reviewers: broulik, Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D3936