Async refresh

Move refresh of google music content to an extra thread.
This fixes two problems:

1. Very long startup times on small systems, like the raspberry pi
2. Remote changes to the content, like new/changed playlists
So apparently "refresh_playlists" is more than for playlists refresh, since library refresh is also called here.

As mentioned in my general comment on #41, I think we need a different caching strategy for library vs playlists vs radios. We might want to refresh radios every hour, while library refresh could be done each day.