WIP: load radio stations

* load radio stations as playlist on start up (this may take some time)
* add logging for playlist loading to show progress

Things to do:
* [x] ~~Load playlist asynchronous. It takes ages on a raspi even without the radios.~~ (see #42)
* [x] ~~Reload radios every now and then.~~ (see #42)
* [x] Let user config the number of tracks (and radios)

This is far from perfect. Please state your ideas.
Could you would do this refactor in another PR?