Create client application for integration tests

[dvzrv]

To run integration tests it would be better to rely on a client application, which communicates with caterpillar over d-bus instead of only testing the outcome of its actions.

With #5 the integration test for "there is no update" does not work anymore, as caterpillar has been turned into a long-running application and the tests relied on it exiting at some point.

[dvzrv]

More specifically the test has now been removed (6ff7f17) and the test strategy has to be adapted to test using the new d-bus interface instead.

A separate client based on an auto-generated caterpillar proxy can be created to interact with the caterpillar service, depending on test scenario (interactive/ non-interactive).