feature: add dbus event listening

[fabian-thomas]

This PR includes some code I wrote some time ago to improve my own spotify polybar module. But I wanted to make it available to more people.

Instead of executing the status script every second, we now listen for dbus signals. This eliminates polling and removes delays when e.g. switching songs.

This is a rather big change. I don't know if this code works with python 2 too, but I guess python 2 support could be dropped soon.