TaskAction: Disconnect task dependent listeners for action.

This commit sets the TaskAction's task to None in .destroy() method,
so that task dependent trait listeners are disconnected.
Unless this is done, TraitChangeNotifyWrapper wrappers are
leaked on creating and deleting TaskAction instances which have
listeners on TaskAction dependent properties.
Merge pull request #81 from pankajp/task-action-destroy-listeners

TaskAction: Disconnect task dependent listeners for action.