Support for controlling MIDI expression (issue 114500045 by address@hidden)

[pkx166h]

This has now been approved, I assume you still have push access so if so
make sure you push to HEAD:staging and not directly to master.

Also, as I indicated in the Tracker, does this require any additional
documentation in the Notation Reference (sections 3.5.x) and if so can
you make a new tracker perhaps or send some suggestion to the bug list
so it gets picked up for the future additions.

https://codereview.appspot.com/114500045/