Support passing callbacks to elements

[jstarry]

Problem

The only way to specify listeners for elements is with the magical closure syntax. That approach is not very flexible and is inconsistent with the syntax so this change allows setting a callback struct as a listener.

New syntax:

use yew::{html, Callback, ClickEvent, Component, ComponentLink, Html, ShouldRender};

struct Model {
    onclick: Callback<ClickEvent>,
}

enum Msg {
    Click,
}

impl Component for Model {
    type Message = Msg;
    type Properties = ();

    fn create(_: Self::Properties, mut link: ComponentLink<Self>) -> Self {
        Model {
            onclick: link.send_back(|_| Msg::Click }
        }
    }

    fn update(&mut self, msg: Self::Message) -> ShouldRender {
        match msg {
            Msg::Click => true,
        }
    }

    fn view(&self) -> Html<Self> {
        html! {
            <button onclick=self.onclick>{ "Click me!" }</button>
        }
    }
}

[hgzimmerman]

This feels like the right way to handle the awkwardness of using link as was discussed in #756.

Edit: specifically the creation of callbacks in create.