In v5, the way extension developers handle preferences being loaded initially was to subscribe to the PreferencesEvent. This PR adds a much better way (so we can remove this event in the future).

It's now done in the Extension.__init__ method. This makes things less confusing for users, without multiple extension init events/methods, and this way the extension will be able to access the preferences as soon as it's created, without delays/waiting.

class MyExtension(Extension):
    def __init__(self):
        super().__init__()
        value = self.preferences.get('my_preference_id')

It is sending the whole preferences object as stringified json in an environment variable, then loads it again. It might be inefficient, but the object is small and should be fast to format and parse.

Checklist

• Verify that the test command ./ul test is passing (the CI server will check this if you don't)
• Update the documentation according to your changes (when applicable)
• Write unit tests for your changes (when applicable)