Seems like subprocess and friends (Popen, communicate) are not so easy
to mock cleanly. Therefore, start from scratch and carefully write test
by test, until (at least) the old test coverage has been restored.
Add an interface that allows arbitrary objects to store/retrieve
arbitrary key/value pairs. This will be used for different purposes in
the future:
* Config class(es) can store user-defined parameters for modules
* Widgets can store state
* ???
see #23