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/widgets/playingwidget.h

@@ -40,8 +40,6 @@
 #include "core/song.h"
 #include "covermanager/albumcoverloaderoptions.h"
 
-using std::unique_ptr;
-
 class QTimeLine;
 class QTextDocument;
 class QPainter;
@@ -98,7 +96,7 @@ class PlayingWidget : public QWidget {
 
   void AutomaticCoverSearchDone();
 
-  void AlbumCoverLoaded(const Song &song, const QUrl &cover_url, const QImage &image);
+  void AlbumCoverLoaded(const Song &song, const QImage &image);
   void SetHeight(int height);
   void FadePreviousTrack(qreal value);