Deprecate event handling with `handle{...}`

[jpsacha]

handle{...} was introduced to simplify creation of event handlers.

Starting with Scala 2.12 handle{...} can be replaced with idiomatic Scala code _ => {...}. For instance, instead of old:

button.onAction = handle {
  println("Handling button action")
  doSomething()
}

we can now write:

button.onAction = _ => {
  println("Handling button action")
  doSomething()
}