Support GPX files

I have made some progress working on this- I have implemented in my local musescore repository something similar to the java implementation shown above with respect to extraction of the compressed format and gathering the files in the file system ready for parsing. The files contained in this file system still need to be parsed so there's still lots of work to be done here, but it's a start. :-)

I'm closing this bug as the implementation has reached the point where we should be getting individual bug reports on features that don't work / display in some unexpected way etc (though please wait for PR #1101 to be merged as it contains a whole pile of features, I would think that will be some time next week). Please do feel free to create issues for individual features after that though! :-)

Automatically closed -- issue fixed for 2 weeks with no activity.