[trunk] Part export

In reference to http://musescore.org/en/node/14578 , here is a patch that allows the user to export all parts in a score to a folder; the user can choose the output format. I created this patch on top of the 2.0 code, so I suspect that is where it needs to go.

Please let me know if there are any changes I need to make. There were a couple decisions I had to make about the export, and I think they're the best default decisions for now.

Thanks!

EDIT: Realized it didn't work if you had the current score open to a part. Fixed with the new patch (and removed previous patch)