applets/kickoff: fix invalid mimeData from item delegates

This fixes "QML KickoffGridDelegate (parent or ancestor of QQuickDragAttached):
Don't know how to encode variant of type QMetaType(QUrl) as mime type "text/uri-list"".

BUG: 485485
FIXED-IN: 6.0.4


(cherry picked from commit ec5b29c0)