Clear image file

Whenever a song changes, we need to clear the path that we tell the
album art image to load.  Otherwise, it will show art from the previous
track if none can be found for the current track.

ocarina/image.py

@@ -30,6 +30,7 @@ class AlbumArt(Image):
 		libsaria.event.invite("POSTGETART", self.set)
 
 	def preload(self, *args):
+		self.file = None
 		self.hide()
 
 	def set(self, file=None):