Include .xml suffix for export
Replacing the .xml with the .musicxml has caused confusion among users. It also make the users who overcome the confusion to have to change the extensions so their files are compatible with their existing projects and programs. Adding the .xml extension as an alternative to the .musicxml extension as far as I know causes no compatibility issues for the users. If there are some issues, then the few cases would need at most 3 lines of code to decide if it is necessary write the version 3.0 info or the older info to match the extension. The older code would still be on github so it would be a matter of copy and paste.