Register widget can't auto register widget systems.

[StarArawn]

Just need to figure out how to grok the types into playing nicely.

[StarArawn]

Just for documentation purposes you can now register widget systems through the widget trait:

impl Widget for WButton {
    fn update() -> impl System<In = (), Out = bool> {
        IntoSystem::into_system(update)
    }

    fn render() -> impl System<In = (), Out = ()> {
        IntoSystem::into_system(render)
    }
}
...
.register_widget::<WButton>()

You can also still use the existing add_widget_systems if you desire. I think the boilerplate for the trait is a bit more heavy handed but its more cohesive.