Implement property for Command.action.

[cvwillegen]

I added a property handler for action, so that it is possible to specify what a Command does after it's been created.

Sometimes, you want to make the action depend on the current state of your application, and having global variables to indicate that state may not always be possible.

With this change, you can change the action to be another closure. But, because the handler needs to be wrapped in a wrapped_handler, doing this from you app looks ugly:

self.removepodcast.action = wrapped_handler(self.removepodcast.action, self.showRemovePodcast(main_list_box, podcast_id)

After this change, it looks like this:

self.removepodcast.action = self.showRemovePodcast(main_list_box, podcast_id)

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the PR! A couple of minor suggestions inline; but this looks like an obvious oversight in the API, so we should definitely fix it.

[freakboy3742]

This test is a little odd; it's only really validating that the action can be cleared, which I think would have always been true.