applets/systemtray: Prefer Icon over IconName
The systemtray has a custom KIconLoader in order to make the SNI's IconThemePath work.
When passing an icon name to Kirigami.Icon we skip that. Instead prefer the Icon role from the model, which goes through the custom loader
This restores the logic to how it was before d31b985b.
BUG: 479712 FIXED-IN: 6.0.1
(cherry picked from commit a713f235)