Player Signal API

    "playing"                    : ( position, duration )
    	This is called when the player starts playing.

    "paused"                     : ( position, duration )
    	This is called when the player pauses.

    "stopped"                    : ( )
    	This is called when a track stops playing.


Playlist Signal API

    "bookmark-added"             : ( parent, position )
        Called when a bookmark is added.
        - parent: the id of the bookmark's parent item
        - position: the position of the bookmark in relation to all the other
          bookmarks associated with the parent item.

    "bookmark-removed"           : ( parent, position )
        Called when a bookmark is removed.
        - parent: 

    "end-of-playlist"            : ( )
    	Called when the end of the playlist is reached.

    "item-moved"                 : ( item, new_position )
    	Called when an item in the playlist is moved.
    	- item: the item in question
	    - new_position: the new position of said item

    "file-queued"                : ( item, position )
    	Called when a new file is queued.
    	- item: the item in question
    	- position: the position of the item

    "file-removed"               : ( item, position )
    	Called when a file is removed from the playlist.
    	- item: the item in question
	    - position: the position of said item

    "new-metadata-available"     : ( metadata )
    	Called when new metadata is available.
	    - metadata: a dict of metadata for the currently playing track

    "new-track-playing"          : ( )
    	Called when a new track begins playing.

    "playlist-to-be-overwritten" : ( )
    	Called when a playlist is about to be over-written. If the function
	    returns True the playlist will be over-written. Otherwise the
	    over-write will be aborted.