Fretboard diagrams: for barres, allow user to specify a curved line (instead of a straight line)

Currently a barre is indicated by a solid line on a fretboard diagram, e.g.

However, another symbol widely used in published works, and online, is a curved line for the barré; this appears just above the position where the solid barre woud have been, e.g.

This can also be used for partial barres, e.g.

Unlike the straight line barre, the curved line barre allows finger dots to be displayed at the same position.

Came up at https://musescore.org/en/node/327036 and #110566: Fretboard Diagrams: Bugs.