Live Assist - Shuffle button for button player column

[radiorob]

Shuffle button so user doesn't have to scroll down to find new material. Only affects buttons that are loaded in LA PL, so they are already in the LA interface.
reducing staleness factor at top of list, find different mix. Some of these button PL will be large, put in a limit of actual buttons per PL to load. eg 20. With a display of 5 columns of buttons this gives us over 100 items (including album art)
Press a button to shuffle the column