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/core/mpris2.h

@@ -39,6 +39,7 @@
 #include <QJsonObject>
 
 #include "engine/engine_fwd.h"
+#include "covermanager/albumcoverloaderresult.h"
 
 class Application;
 class Song;
@@ -204,7 +205,7 @@ signals:
   void PlaylistChanged(const MprisPlaylist &playlist);
 
  private slots:
-  void AlbumCoverLoaded(const Song &song, const QUrl &cover_url, const QImage &image);
+  void AlbumCoverLoaded(const Song &song, const AlbumCoverLoaderResult &result = AlbumCoverLoaderResult());
   void EngineStateChanged(Engine::State newState);
   void VolumeChanged();