Several changes:

-Update song generator jar to include some new instruments, a new way to
generate notes, and some refactoring around MIDI instruments
-Change code to handle refactoring of instruments
-Update/modify UI strings (shorten the word INTRUMENT for better
display)
-Handle case where user has not generated a song yet but tries to export
to MIDI (show error and take no action to try to save)
-Display message to user that file was saved to music folder upon
successful save.