MediaPlayer: add previous and next track function for optical disks (enhancement)

[vidrep]

Currently the ability to select the previous or next track on a audio CD is only supported once the disk icon is dragged and dropped onto MediaPlayer, creating a playlist. This feature should also be supported when opening a disk or directory from Tracker. See attached MediaPlayer.png

[humdinger]

Could be done like ShowImage deals with images in the folder of the image it was started with (or that was dropped on it). So, not just for files on an optical disk, but as a general feature. Of course, if there's a playlist, that one takes preference.