Introduce the concept of primary and secondary weathers:
* Primary weather keeps the same behavior as previously across all weather providers, so it's non-breaking. This location is not necessarily the current location, just the primary weather location set by the user.
* The GenericWeatherReceiver now has a new extra WeatherSecondaryJson, that receives a json list with secondary weather locations.
It's guaranteed that the primary weather always exists, so the list of WeatherSpecs provided to devices is never empty. Update all support classes accordingly.
TODO:
- also reconstruct json for Pebble background js fake replies
- find a better location for settings
- interatively display candidates when looking up location
- grey out setting on non-cm/los devices
