During various passes, widgets that want to signal a semantically meaningful event from the UI (eg a button that was clicked) can emit actions.

Actions are a centralized queue of messages intended for the root application, mostly for the app driver (eg Xilem) to handle. They are inspired by Facebook's Flux architecture.

Actions are currently under-designed. Right now, they only store the emitter's id, plus a payload among a small set of types with Box<dyn Any> as a wildcard:

pub enum Action {
    /// A button was pressed.
    ///
    /// Some presses are triggered without a pointer button;
    /// for example, a touch screen does not exercise buttons.
    /// In these cases, `None` will be the value here.
    ButtonPressed(Option<PointerButton>),
    /// Text changed.
    TextChanged(String),
    /// Text entered.
    TextEntered(String),
    /// A checkbox was toggled.
    CheckboxToggled(bool),
    // FIXME - This is a huge hack
    /// Other.
    Other(Box<dyn Any + Send>),
}

Ideally, we'd want each widget to have its own action variants, so the Widget trait should have an Action associated type.

This associated type be bound of Self: Sized so that the trait remains dyn-safe.