Move idle, external, and similar features elsewhere

Created by: ederag

"External" was never brought back (#271 (closed)), and notifications are not working at all (tested on openSUSE 15.1). There have been related feature requests (#123) and PRs (#118, #259) pending for a long time.

No wonder: that kind of features is nice to have, but hard to test, and increase a lot the maintenance burden and responsibility.

All these features can be implemented separately, talking to the main hamster through dbus, for instance.

Let's remove them from the main repo, and if there is an interest, eventually delegate that to a new separate project (say hamster-companion), preferably with another maintainer. [I would gladly help to set it up, of course, so please ask here]

That would even be reusable with the rewrite if it resumes.