Feature request

There should be a python module importable by import packagename.action.actioname which contains everything needed to use actions in python.

Feature description

The module needs to have references to all the types a user needs to interface with actions. This should follow the same model as services since a service type support includes two message type supports.

Implementation considerations

The service python object imports the generated python objects for the request and response message. The action module should import the generated python objects for the services and message it uses as well as the common cancel service and status message.

The python object for services is just a class with two class members: Request and Response. Actions could be a class with 3 members: SendGoal, GetResult, and Feedback

connects to ros2/ros2#583