libgweather shows unexpected icons

In libgweather_2.24.1 and older, the rationale for using icons representing weather conditions is flawed. See http://www.nws.noaa.gov/glossary/index.php and http://www.deutscher-wetterdienst.de/lexikon/index.htm for references.

1. Haze has a horizontal visibility range of between 2 km and 8 km; Mist has between 1 km and 2 km. Both are now represented with icon weather-fog. This is clearly not what anyone would expect. They should not be represented, using cloud cover instead.
2. A funnel cloud equally is not a visibility impediment that could be understood as fog. It should be represented as weather-storm, like a tornado, which it only differs from in that it does not touch the ground.
3. Broken cloud covers are defined as a sky obscuration of between 5 and 7 octas (eigths), which means between 56 % and 94 % of the sky is covered. At present, this is represented with weather-few-clouds (weather-few-clouds-night resp.) This is not a fair representation of real circumstances. As long as weather-cloudy is not available as a distinct icon, weather-overcast should be used. This matches current praxis in the Gnome and Tango icon libraries, where stock-weather-cloudy redirects to weather-overcast.

I have added a patch, please excuse me if the form isn't perfect, I am doing this for the first time.