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/collection.cpp

@@ -158,6 +158,7 @@ void SCollection::ResumeWatcher() { watcher_->SetRescanPausedAsync(false); }
 void SCollection::ReloadSettings() {
 
   watcher_->ReloadSettingsAsync();
+  model_->ReloadSettings();
 
 }