applets/kicker: Port the remnants of QIcon to string icon names

Amends f6628194 and fixes Drag&Drop all over the kicker / dash applets.

Test plan

plasmoidviewer -a org.kde.plasma.kicker
plasmoidviewer -a org.kde.plasma.kickerdash

Drag the various delegates around everywhere.

Bugs fixed

Not sure. Couldn't find any matching bug report.