I often search for a song or have a song playing, that I want to be added to a 
specified playlist. Add an option to add a song to a specified playlist.

In the web interface, add a new link, "++", "add to specified playlist" 
(change the existing link to "add to current playlist"). This link shall exist 
in both frames ("song") and player. When used, a dialog is presented with the 
possibility to select one of the existing playlists, or alternatively specify 
a new one, and then "Save".

On the player, use e.g. "press and hold + (add)" to go to the corresponding 
dialog, select a playlist, right button to save.