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/playlist/playlistitem.h

@@ -49,7 +49,6 @@ class PlaylistItem : public std::enable_shared_from_this<PlaylistItem> {
   virtual ~PlaylistItem();
 
   static PlaylistItem *NewFromSource(const Song::Source &source);
-  static PlaylistItem *NewFromSongsTable(const QString &table, const Song &song);
 
   enum Option {
     Default = 0x00,
@@ -87,6 +86,8 @@ class PlaylistItem : public std::enable_shared_from_this<PlaylistItem> {
   qint64 effective_beginning_nanosec() const { return HasTemporaryMetadata() && temp_metadata_.is_valid() && temp_metadata_.beginning_nanosec() != -1 ? temp_metadata_.beginning_nanosec() : Metadata().beginning_nanosec(); }
   qint64 effective_end_nanosec() const { return HasTemporaryMetadata() && temp_metadata_.is_valid() && temp_metadata_.end_nanosec() != -1 ? temp_metadata_.end_nanosec() : Metadata().end_nanosec(); }
 
+  virtual void SetArtManual(const QUrl &cover_url) = 0;
+
   // Background colors.
   void SetBackgroundColor(short priority, const QColor &color);
   bool HasBackgroundColor(short priority) const;