Implement disk caching of album art (#360)

* Implement disk caching of album art

This includes a button to clear the cache in the settings, as
requested.

Closes #358

* Make the cache size defaults match

* Implement the review by jonaski

* Fix more problems with the PR

src/collection/collectionbackend.h

@@ -188,6 +188,8 @@ class CollectionBackend : public CollectionBackendInterface {
   SongList GetSongsBySongId(const QList<int> &song_ids);
   SongList GetSongsBySongId(const QStringList &song_ids);
 
+  Song::Source Source() const;
+
  public slots:
   void Exit();
   void LoadDirectories();