applets/systemtray: Prefer IconName over IconPixmap

In case both IconName and IconPixmap are provided, the system tray
applet is going to prefer the pixmap.

That can create a dark icon on dark background bug because plasma can't
recolor pixmaps.

The SNI spec recommends visualizations to prefer icons over pixmaps:

> An icon can either be identified by its Freedesktop-compliant icon
> name, carried by this property of by the icon data itself, carried by
> the property IconPixmap. Visualizations are encouraged to prefer icon
> names over icon pixmaps if both are available (FIXME: still not very
> defined: could e the pixmap used as fallback if an icon name is not found?)

BUG: 418996
(cherry picked from commit 7363d0e0)