Load default icon using Gtk.IconTheme

We are using an absolute path before, which doesn't take into account the
current theme and icon path.
albumArtCache: Move to new class the loading of album art from uri

Create a new class GetUriRequest which would download the image from the uri.
albumArtCache: Merge lookup and lookup_or_resolve

Merge lookup and lookup_or_resolve into lookup, then move their callbacks to a
new class.
Update callback calls for latest changes in album art loading
Merge pull request #129 from kyoushuu/album-art-cache-rewrite

Album art cache rewrite