 Scala Swing event-system documentation

Official source: github.com/scala/scala-swing

Relevant locator: Official library README, About Scala-swing: Publisher/Reactor, listenTo, reactions; archived Reactor API at www.scala-lang.org/api/2.12.7/scala-swing/scala/swing/Reactor.html corroborates listenTo, deafTo and reactions. These corroborate the supplied source contract, not a verified 2021 course dependency version.