GWeather only works with GTK 3.x

I just started porting Maps to GTK 4. Then I realized that we use GWeather (e.g. to look up the nearest city to a coordinate, when sharing places with Clocks and Weather, and also to get the country/region of a given coordinate to lookup address formats and the driving side, used to pick icons for roundabout instructions for routing).

And GWeather has a dependency of GTK (and currenly only builds a GTK 3 variant). I think options might be to either build separate libraries for GTK 3 and 4 (I think WebKit-GTK does this), or maybe better split out the GTK widgetery into a separate sublibrary and have a non-GUI base library (the functionallity we use in Maps are not dependent on the GTK widget, btw).