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?)
Edited by Nate Graham