Musicxml handling

[gb0808]

This allows MUSIC XML files to be uploaded as projects in NetsBlox. The code is sloppier than I would like it to be. I will refactor it once the demos have been submitted.

[brollb]

Given the urgency for the tools competition, I am going to go ahead and merge this. However, there are a few issues that will need to be addressed later (in another PR):

• It would probably be better to create the objects programmatically in-memory then call toXML instead of generating the XML here. It would be easy to introduce bugs here that are not easy to catch due to the subtle relationship between the toXML methods and the logic here. Instantiating objects would still be coupled to the constructors but that dependency is easy to catch since changes to a constructor's signature always requires consideration of where it is invoked.
• The generated code can only be opened by an instance of NetsBlox with the BeatBlox extension loaded. This isn't great since this means that code in the core project is coupled to a specific extension.

I added a few smaller comments below.