Staff: expose _userDist member variable in Staff Properties dlg box.

This allows to modify the extra user distance above the staff via numeric input  in addition to mouse dragging operations.
fix #20823: Possible access violation
Instruments.xml update

Plucked instruments and vocals are now XML 3 compatible. XML syntax
checked - all seems to be well
Improve error message on import of Capella 2.x files
Improve spacing of "/" in renderBase

Also restore original y position after rendering bass note.
Merge pull request #304 from ChurchOrganist/InstrumentsXMLUpdate

Instruments.xml update
Merge pull request #302 from mgavioli/STAFF_expose_user_distance

Staff: expose _userDist member variable in Staff Properties dlg box.
Merge pull request #303 from AntonioBL/fix_possible_access_violation

fix #20823: Possible access violation
Merge pull request #307 from Jojo-Schmitz/Capella_2.0

Improve error message on import of Capella 2.x files
Merge pull request #308 from softins/stdchords-tweak

Improve spacing of "/" in renderBase
Merge pull request #309 from trig-ger/midi_separation

Prototype of left/right hand midi track separation