applets/systemtray: Prefer symbolic icons
Prefer icon names to QIcons, also if an icon name is provided and doesn't have the -symbolic suffix, add it. This should make more monochrome icons, also in non-breeze themes (no glaring visual differences on breeze)
This won't adress XEmbed-sniproxy icons and won't address also icons that only send a pixmap instead of an icon name, but those are cases we can't do anything about