Add support for Unity badges

[danirabbit]

Fixes #74
Fixes #201

Tested with Granite Demo. The DBus class is taken almost directly from Plank with code style fixes.

[jeremypw]

This worked with granite-demo (and App Center notifications) but not with granite-7-demo. However, the existing Plank dock did work with granite-7-demo.

[leolost2605]

I'm not sure but I think this might be a problem with granite demo. The application id set there is io.elementary.granite.demo which is used by granite to send the unity entry but the desktop file is io.elementary.granite-7.demo which is what gala and therefore dock know about. Plank only works because it adds a second app for the unity entry which has no icon etc. I'm not sure though what the preferred behavior here is. IMO it's better to hide it entirely (like it's done in this PR)

[danirabbit]

Yeah I think the correct behavior is to make sure we're getting a consistent ID so that apps can't pretend to be other apps etc

[leolost2605]

Looks good only one minor nitpick :)