support for shuffled music playlist . Attempt at https://gna.org/bugs/?19653

This is my first attempt at https://gna.org/bugs/?19653 .

What I did to implement the desired functionality:
- added code to treat another key in the [music] tag, namely "shuffle". Possible values are "yes", and "no", any other value is completely ignored. Whenever you add the shuffle key with a value of "no", the current track list will play in order. The default behavior is to shuffle.
- replaced the local variable "track" with a global track counter; this counter is incremented only when shuffle is off, and choose_track() is called. This way, every time a new track is to be chosen, the "next" track in the list is played. The counter resets when the entire list has been played.

Just to illustrate the functionality, I decided to add the "shuffle" key in the default music playlist macro. I'll remove it eventuality.

Any kind of feedback or advice on what could be done to improve this would be greatly appreciated.

Thanks in advance!