Correctly align non six string fretboard diagrams

Fretboards with fewer or more than six strings are not correctly aligned above notes.

Steps to reproduce: Insert note in score; attach fretboard diagram; edit fretboard diagram properties to have fewer or more than six strings.

Expected behavior: Diagram is centered over note.

Actual behavior: Diagram is placed where it would be aligned if strings were added or subtracted on the right side to make six strings.

Version: 6347ed6

OS: Mac OS X 10.11.6