The former artworkcache was a monolithic method filled with callbacks,
hard to debug and comprehend. It also left quite a bit for the caller to
take care of.

The current design is that Music has an Art object that is specific to
one cairo.Surface or Gtk.Image (ArtImage) as requested. The Art object
takes care of retrieving the correct image and emits a signal
(cairo.Surface) or updates the Gtk.Image when done. This leads to less
art related code in the views and widgets.

The lookup process itself is now clearly divided into several steps:
1. (Cache) libmediaart cache lookup
2. (EmbeddedArt) local lookup
    1. tags (gstreamer)
    2. coverart in the directory (libmediaart)
3. (RemoteArt) remote lookup through Grilo coverart providers

Using a cairo.Surface in the Gtk.TreeView pixbuf renderer also allows
for HiDPI art in SearchView.

For simplicity and cleanliness, all art related calls have been removed
from BaseView as they were unused and there is no plan to bring it back
to BaseView.

Closes: #65