Don't return QIcons for Kicker models

When the models return QIcons, the result is icon data that's recolored
by KIconLoader according to the system color scheme. However these
returned QIcons may be displayed by Kirigami.Icon instances in Kickoff
and other applets using a different Plasma color scheme from the
systemwide one, and then the icons will be colored incorrectly.

We can fix the issue by removing usages of QIcon and instead returning
the icon names or file paths directly; Kirigami.Icon can handle them
both internally and perform the recoloring correctly for all plasma and
systemwide colorscheme combinations.

BUG: 472864