music: abstract the functions used to control music player

We still use Spotify but we isolate the music functions into a
specific module. Other music player would implement the same
interface.
music: also allows one to use modal keys to control music player

By using mod+s, we switch to a local keymap that control the music
player. The keys are then the same as in XMMS.
music: `Play` method of Spotify seems buggy, just use `PlayPause` instead