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.cpp

@@ -899,6 +899,10 @@ SongList CollectionBackend::GetCompilationSongs(const QString &album, const Quer
 
 }
 
+Song::Source CollectionBackend::Source() const {
+  return source_;
+}
+
 void CollectionBackend::UpdateCompilations() {
 
   QMutexLocker l(db_->Mutex());