Event capturing

[hecrj]

This PR allows widgets to inform the runtime about whether they have handled an event or not.

Specifically, a new event::Status enum has been introduced. It has two variants: Ignored and Captured, and Widget::on_event now must return one of them:

• Captured if the event was handled by the implementor.
• Ignored otherwise.

As a consequence, UserInterface::update has been changed to return an event::Status for every processed Event. This should help users that are integrating iced with their own custom event loop to implement proper event propagation (see #408 and #455).

Furthermore, subscription::Recipe in iced_native now takes a stream of (Event, event::Status) as input, effectively allowing subscriptions to discard events that have already been processed by a widget. This is specially useful for implementing global hotkeys (e.g. if a TextInput is focused, hotkeys will not trigger when typing on it).

Fixes #408.
Closes #455.