fix always using track replaygain because of typo
fix race conditions

 * libgroove no longer auto-consumes files. closes #5
 * there is no groove_player_next. use groove_player_seek. closes #7
 * groove_player_seek_rel removed
 * groove_player_seek_abs renamed to groove_player_seek. closes #4
 * there is no stop state. a player is either paused or not.
 * nowplaying event includes a pointer to the old playlist item as well
   as the new playlist item, and it reflects the true nowplaying
   state rather than the "nowdecoding" state. closes #6