Add setting for length of Server Shuffle playlist

[eharris]

I use the Server Shuffle mode a lot, and it would be nice if there were a setting that let you specify the length of the playlist generated when using that. It seems to always default to 100 items, and I would often like to do 200 or 300 so that when I edit some tracks out, I still have a good length playlist. But it might also be nice to have only a 20 item or 50 item playlist sometimes.