Add Lucca (Italy)

Missing cities in Italy were deferred because I want to reorganize
them to include proper provinces and nothing else, but this isn't
happening any time soon, so add it for now...
Move the timezone cache to GWeatherWorld

Where it rightly belongs
Fix gweather_location_find_nearest_city

Add a location parameter to limit the search, and use the standard
lat/lon abbreviations. Also add warnings to the documentations
for the risks associated with this function and geolocation.