Improvements to the clementine package.

 -  Pass the gstreamer plugins as a list attribute instead of
    individual attributes.
     +  This should allow for greater customization (e.g.: a
          user can add / remove plugins by overriding
          the plugin list attribute).

    This is the same technique as used for wrapping chrome
    with gstreamer plugins.

 -  Moved integration of gstreamer plugins, spotify
    library and clementine into a wrapper derivation.
     +  This should allow a user to change the gstreamer
          plugin set and activate / deactivate the spotify
          plugin without triggering a whole rebuild of the
          clementine sources (which is pretty long).