Allow manual fret number spec for natural and artificial harmonics in tabulature

This issue pertains to linked notation and tabulature staves, as used in guitar and electric bass music. MuseScore automatically determines the correct fret numbers according to the tuning specified in the instrument settings. The fretboard positions for natural and artifical harmonics, however, are different and automatic fret numbers may be incorrect.

Proposed solution: Provide a way to manually override displayed fret numbers on tabulature staves. This should not affect playback.

Additional considerations: Artificial harmonics are even more complex. In the basic case one will need to specify the fret number that is stopped down and the fret number that is pinched or touched in order to produce the harmonic. Maybe this should become a separate issue.

See discussion under Natural Guitar Harmonics in tab