An Acts trait

[sowbug]

The idea is a replacement for the old show(ui) -> (Response, SomeCustomAction) pattern, which sucked. New one is just to implement Displays, but also have a field that contains the action that should be dealt with after calling ui(). The trait would look something like this:

trait Acts {
  type Action;

  // Returns the pending action, if any, and resets it to None.
  fn take_action(&mut self) -> Option<Action>;
}

// callers
let response = entity.ui(ui);
if let Some(action) = entity.take_action() {
  // deal with it
}