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/core/application.h

@@ -126,6 +126,7 @@ signals:
   void SettingsChanged();
   void SettingsDialogRequested(SettingsDialog::Page page);
   void ExitFinished();
+  void ClearPixmapDiskCache();
 
  private:
   std::unique_ptr<ApplicationImpl> p_;