Task: provide API to cancel a task

This PR add an API for task cancellation. It contains:

• Task.cancelled getter / setter -> allows to knows if the task has been cancelled
• cancel function: function to be called if the task is cancelled. This is at each task to define the way to handle cancellation. the cancelled attribute must be set during call to cancel function. note: This will most likely not be implemented by most tasks. But in the case a user need to be able to cancel a task then this is the entry point.