Browse Artists and Albums

Let user browse the existing/cached library.
No network IO calls done during browse.

Directory Structure:

* Google Music
  * Albums
    * Tracks
  * Artists
    * All Tracks
      * Tracks
    * Albums
      * Tracks

TODO:

* [x] fix sorting albums, e.g. `disc_no`
* [x] use `_lookup_*()` to reduce code duplication and show aa content