Abortable Task

[hecrj]

This PR introduces the Task::abortable method, which returns a task::Handle that can be used to easily cancel a Task that has already been handled to the runtime.

This is useful for long-running tasks that are not well-suited to become subscriptions (i.e. may eventually finish on their own) and that may need to be canceled by the user, like downloading a file (yes, this means our download_progress example is outdated).

[JL710]

What would the key difference between Tasks and Subscriptions be?

[hecrj]

@JL710 The main difference is the guarantees they provide. From an application's perspective:

• A Task is some workload that may eventually finish on its own, independently of the application state.

• A Subscription is a request to listen to "something" that must always be there for the whole lifetime of the application (like a webserver connection, the iced runtime, or just time itself). A Subscription cannot end on its own; only the application can eventually unsubscribe from it.

Therefore, all subscriptions are infinite and abortable by design; and aborting can be done declaratively—the same way that widgets can be shown and hidden in view.