Fixes to playlist context menu

- Add all playlist actions to initialization list
- Make rescan songs work for non-collection songs by using playlist item reload
- Only show add to another playlist and remove from playlist when songs are selected
- Add some missing icons

Fixes #503

src/core/mainwindow.h

@@ -346,8 +346,6 @@ class MainWindow : public QMainWindow, public PlatformInterface {
   QAction *playlist_add_to_another_;
   QList<QAction*> playlistitem_actions_;
   QAction *playlistitem_actions_separator_;
-  QAction *search_for_artist_;
-  QAction *search_for_album_;
   QAction *playlist_rescan_songs_;
 
   QModelIndex playlist_menu_index_;