metadata/gps: City detection from 100 to 15 km

The original idea was that it's helpful to at least know the very rough
location of an image. In practice that mostly seems to be confusing. So
reducing this to a much smaller distance.

Closes #318

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/370>

src/metadata/gps.rs

@@ -145,8 +145,8 @@ impl GPSLocation {
                 self.longitude.to_f64(),
             );
 
-            // do not use city if more than 100 km away
-            if location.distance(&location_exact) < 100. {
+            // do not use city if more than 15 km away
+            if location.distance(&location_exact) < 15. {
                 if let (Some(city), Some(country)) = (location.city_name(), location.country_name())
                 {
                     return format!("{city}, {country}");