Invert settings Tunable <--> Widget dependency relation to improve code reusability

Make settings widgets hold information about their tunables instead of the other way around.
Tunables are now independent of their presentation/rendering in the menu. Different widgets can be used to represent tunables of the same type.

E.g.: WiFi SSID and WiFi password are both stored as the same string tunable type, but are rendered different via different widgets.