process embedded images received during playback

- add a new signal on the RBPlayer interface to make embedded images
  available as GdkPixbufs
- move tag processing code to rb-player-gst-helper.c, add code to handle
  converting images from GstBuffers to GdkPixbufs
- process GST_TAG_IMAGE and GST_TAG_PREVIEW_IMAGE tags, emitting the new
  'image' signal
- emit rb:coverArt extra metadata notification when an image is received
  from the player, causing the art display plugin to use the embedded
  image.

  Fixes half of #345975.