spotify: try to support the new version of Spotify

Use dbus-mediakeys to get an appropriate DBus interface. In future
versions, this should not be needed anymore. See:
 https://github.com/vincentbernat/dbus-mediakeys

rc/start.lua

@@ -17,7 +17,9 @@ local execute = {
       "text/html",
    -- Default MIME types
    "xdg-mime default evince.desktop application/pdf",
-   "xdg-mime default gpicview.desktop image/png image/x-apple-ios-png image/jpeg image/jpg image/gif"
+   "xdg-mime default gpicview.desktop image/png image/x-apple-ios-png image/jpeg image/jpg image/gif",
+   -- dbus-mediakeys
+   "dbus-mediakeys &"
 }
 
 os.execute(table.concat(execute, ";"))

rc/tags.lua

@@ -43,6 +43,10 @@ shifty.config.tags = {
       screen = math.max(screen.count(), 2),
       icon = tagicon("im"),
       nopopup = true,           -- don't give focus on creation
+   },
+   spotify = {
+      screen = 1,
+      exclusive = true
    }
 }
 
@@ -60,6 +64,10 @@ shifty.config.apps = {
       match = { class = { "Skype", "Pidgin" } },
       tag = "im",
    },
+   {
+      match = { class = { "Spotify" } },
+      tag = "spotify"
+   },
    {
       match = { config.termclass },
       startup = {