Use the notifier type in the task class/module directly

Instead of having code that is some what like the notifier
code we already have, but is duplicated and is slightly different
in the task class just move the code that was in the task class (and
doing similar actions) to instead now use a notifier that is directly
contained in the task base class for internal task triggering of
internal task events.

Breaking change: alters the capabilities of the task to process
notifications itself, most actions now must go through the task
notifier property and instead use that (update_progress still exists
as a common utility method, since it's likely the most common type
of notification that will be used).

Removes the following methods from task base class (as they are
no longer needed with a notifier attribute):

- trigger (replaced with notifier.notify)
- autobind (removed, not replaced, can be created by the user
            of taskflow in a simple manner, without requiring
            functionality in taskflow)
- bind (replaced with notifier.register)
- unbind (replaced with notifier.unregister)
- listeners_iter (replaced with notifier.listeners_iter)

Due to this change we can now also correctly proxy back events from
remote tasks to the engine for correct proxying back to the original
task.

Fixes bug 1370766

Change-Id: Ic9dfef516d72e6e32e71dda30a1cb3522c9e0be6