fix #39876: no undo of chord symbol edit
improve rhythmic notation for drum staves
Merge pull request #1476 from MarcSabatella/39876-undo-chord-edit

fix #39876: no undo of chord symbol edit
Merge pull request #1477 from MarcSabatella/accent-notation

improve rhythmic slash notation for drum staves
fix #20783: add section breaks to joined scores

and line and page breaks too...