Event subscriptions

[hecrj]

Closes #29, closes #109, and fixes #123.

This PR adds support for event subscriptions. Event subscriptions are inspired by Elm's subscriptions and can be used to listen to external events that do not necessarily occur as a response to some interaction or command. For instance, you can use event subscriptions to create a websocket connection, listen to incoming messages, and even handle reconnects graciously.

Event subscriptions are built on top of futures::stream::Stream. The messages produced by a subscription are provided to Application::update and, of course, can be used to change the state of the Application.

The iced_winit shell keeps a subscription alive as long as the user returns it in the new Application::subscription method. Therefore, unsubscribing simply consists in not returning the subscription anymore.

Moreover, subscriptions can be created by libraries and/or end-users using the new subscription::Recipe trait (name subject to change), which requires just a couple of methods.

For now, there is only one built-in Subscription in iced_native: subscription::events. This subscription will notify an application of all the runtime events that occur, producing Event values. It can be useful if you want to listen to keyboard or mouse events (and most likely window events soon):

Two examples have been added that showcase event subscriptions:

• events, which uses the new built-in subscription::events.
• stopwatch, which showcases a custom event subscription that produces messages periodically at a configurable time interval.

[clarkmoody]

Demo app works on my system, thanks!

Testing my reflexes now.