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.

lib/spotify.lua

@@ -0,0 +1,35 @@
+-- Drive spotify
+
+local awful     = require("awful")
+
+module("vbe/spotify")
+
+-- Send a command to spotify
+local function spotify(command)
+   awful.util.spawn("dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotify " ..
+      "/org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player." .. command, false)
+end
+
+function playpause()
+   spotify("PlayPause")
+end
+
+function play()
+   spotify("Play")
+end
+
+function pause()
+   spotify("Pause")
+end
+
+function stop()
+   spotify("Stop")
+end
+
+function next()
+   spotify("Next")
+end
+
+function previous()
+   spotify("Previous")
+end