Handle "Too many requests" status code and inform user they can add their own API key

New features:

Add status code handling in the Observer
If status code is 429, add a toast message
If the user clicks on "HELP" button, a dialog is opened and inform the user they can add their own API key. If the user click on the message, this will open the Service provider settings.

Possible improvements:

Before telling the user to add their API key, add a message "Instructions to get a personal API key". By clicking on it, this would open a wiki page on this GitHub telling him how to proceed for each provider.
I didn't know how to proceed with the PollingUpdateHelper, so the API limit toast message never shows up. However, I thing Polling Updater is what runs in the background, so there is never a toast message anyway, right?
Currently, this doesn't seem to inform the user of API limit reached on the location search side (only on weather data fetch), as I couldn't find where it was done
Instead of opening "Service provider settings", we could open directly "Advanced provider settings", but it would require adding a few classes.
This code could serve as a base for #244. However, I'm currently getting the status code from the HttpException and not from the Response itself (which would require heavier refactoring).

WangDaYeeeeee / GeometricWeather