OWEwoksWidgetWithTaskStack: provide API to cancel a task

[payno]

Today OWEwoksWidgetWithTaskStack doesn't provide API to cancel a task.

There is two approaches:

• simple: allow to cancel the current task and the clear all task
• advanced but more powerful: allow to cancel a specific task (like using an id)

[payno]

This was discussed during the ewoks meeting.
Decision was to go to provide future & associated API.

• OWWidgetBase.execute_task() -> OWFuture
• OWFuture.cancel_or_abort(): bool
• In case of "nothread": the OWFuture.cancel_or_abort() always returns False