applets/weather: use symbolic icons when in panel
Currently this code produces no changes when using the Breeze icon theme since it lacks any -symbolic variants of these icons; see https://bugs.kde.org/show_bug.cgi?id=403833. However it will make the icons symbolic when using an icon theme that does have -symbolic variants of these icons. And it will do the same automatically once Breeze gets them too. CCBUG: 403833 CCBUG: 473278