There is now a Temperature struct in the weather service, which holds
the internal representation. There is also a temperature struct in the
Applications namespace, which holds the temperature in either Celsius or
Fahrenheit.
This new implementation of the weather feature provides a new BLE API and a new weather service.
The API uses a single characteristic that allows companion apps to write the weather conditions (current and forecast for the next 5 days).
The SimpleWeather service exposes those data as std::optional fields.
This new implementation replaces the previous WeahterService.
The API is documented in docs/SimpleWeatherService.md.
The DisplayApp class isn't used in the Screen base class and most
screens, so requiring it is pointless.
In this commit, DisplayApp pointers were added to screens which use it
and the explicit Screen constructor was removed in those screens.
