WIP: load radio stations as playlist

* load radio stations as playlist on start up (this may take some time)
* add logging for playlist loading to show progress

Things to do:
* [ ] Load playlist asynchronous. It takes ages on a raspi even without the radios.
* [ ] Reload radios every now and then.
* [ ] Let user config the number of tracks (and radios)

This is far from perfect. Please state your ideas.