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 colorscheme. 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. Simpler code, fewer bugs. Hooray!
I made sure to test absolute filepath images, and those still work.
| Before with Breeze Twilight | After with Breeze Twilight |
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