:lipstick: Clarify names
Don't call super in Behavior

I think the subscription logic in Behavior is nuanced enough that it
makes sense for it to just handle it itself.
Simplify the interface of the Signal back to a single function that is
called before the first value subscription. Anything fancier can be
achieved by subscribing to 'value-subscription-added' events directly.
Use ::subscribe to subscribe to behavior value subscriptions

The first-value-subscription-will-be-added event can be fired multiple
times if all subscriptions are removed and then a new subscription is
added. Using ::subscribe on ourselves ensures we will unsubscribe so we
don't end up with multiple subscriptions.
0.14.0