Major refactoring.

  * Remove song format knowledge from both Render and Vimus
  * Song format changes are distributed by events
  * Parametrize ListWidget by elements rendering format
  * Parametrize Renderable by rendering format
  * All stuff is rendered with format ()
  * Except MPD.Song, which is rendered with SongFormat

These changes let all state to be in widgets and not in
application state/renderer environment

Still not the best (complex logic of maintaining format between
move-in/move-out actions on list windgets is needed), but a bit better than it
was.