Improve album cover loader, lyrics search and streaming support

- Improve album cover loader
- Add album cover loader result struct
- Move album cover thumbnail scaling to album cover loader
- Make init art manual look for album cover images in song directory
- Make album cover search work for songs outside of collection and
  streams
- Make album cover search work based on artist + title if album is not
  present
- Update art manual in playlist for local files, devices and CDDA
- Make lyrics search work for streams
- Add stream dialog to menu
- Remove dead code in InternetSearchModel
- Simplify code in InternetSearchView

src/covermanager/albumcoversearcher.h

@@ -36,6 +36,7 @@
 
 #include "albumcoverfetcher.h"
 #include "albumcoverloaderoptions.h"
+#include "albumcoverloaderresult.h"
 
 class QWidget;
 class QStandardItem;
@@ -88,7 +89,7 @@ class AlbumCoverSearcher : public QDialog {
  private slots:
   void Search();
   void SearchFinished(const quint64 id, const CoverSearchResults &results);
-  void ImageLoaded(const quint64 id, const QUrl &cover_url, const QImage &image);
+  void AlbumCoverLoaded(const quint64 id, const AlbumCoverLoaderResult &result);
 
   void CoverDoubleClicked(const QModelIndex &index);