Add shuffle button for queued songs

[therake]

Currently songs added in a queue are played in the order they are added. It is possible to change the order by going in the queue and moving the songs one by one. However, the most common case is to add a large number of songs, usually going from artist to artist, which makes the queue quite monotonous. It is tedious to try to shuffle the queue by hand by dragging a song at the time. It would be convenient to have a button to reshuffle the queue in one step.