Add setting for configuring the color for the currently playing song

Fixes #676

src/playlist/playlistview.h

@@ -38,6 +38,7 @@
 #include <QString>
 #include <QImage>
 #include <QPixmap>
+#include <QColor>
 #include <QRect>
 #include <QRegion>
 #include <QStyleOption>
@@ -297,6 +298,8 @@ class PlaylistView : public QTreeView {
   DynamicPlaylistControls *dynamic_controls_;
   RatingItemDelegate *rating_delegate_;
 
+  QColor playlist_playing_song_color_;
+
 };
 
 #endif  // PLAYLISTVIEW_H