[KBalooRolesProvider] Support properties of type QStringList

QVariant::toString() unwraps a QStringList with a single string to a QString, however a list with multiple entries returns a null string.
Explicitly check for KFileMetaData property type and convert accordingly.

BUG: 395033
FIXED-IN: 18.04.3

Differential Revision: https://phabricator.kde.org/D13349