[applets/systemtray] Set list sizes correctly

1. Use Kirigami, not PlasmaCore, since this is a config window
2. Remove PlasmaCore import now that it's no longer used
3. Set implicitWidth/height on Kirigami.Icon, not width/height
4. Use SmallMedium icon sizes, which is typical for list items like this

(cherry picked from commit e43c766d)