gweather-weather.c: Add an icon name for tornados

GWeather currently selects the "storm" icon for a broad range of weather. However Adwaita's storm icon (and the icon from many other icon sets) depicts lightning here.

I think it would be safe (because of icon name fallbacks) to add an icon name weather-storm-tornado so that icon themes (like elementary) can add a more accurate icon for Tornados, Squalls, and Funnel Clouds, but Adwaita can continue to use weather-storm if it so chooses