enable selecting and add sel. menu in media lib

Previously, selecting entries was only possible in the "songs" column of
the media library. Now it works in the "artists" and "albums" column as
well, allowing one to add whole albums and artist collections to the
playlist at specific positions via the "add selection" menu.

src/media_library.h

@@ -52,8 +52,8 @@ class MediaLibrary : public Screen<Window>
 		
 		virtual MPD::Song *CurrentSong();
 		
-		virtual bool allowsSelection() { return w == Songs; }
-		virtual void ReverseSelection() { Songs->ReverseSelection(); }
+		virtual bool allowsSelection() { return true; }
+		virtual void ReverseSelection();
 		virtual void GetSelectedSongs(MPD::SongList &);
 		
 		virtual void ApplyFilter(const std::string &);