[trunk] grace notes don't react to all beam properties settings

This is just a matter of playing around with the feature.

Create four 1/16 grace notes.
- Drag "Beam start" onto the 3rd note. Nothing happens.
- Drag "No beam" onto any of them. Nothing happens.
- Drag "Beam 32nd sub" onto the 3rd note. That works. Undo. It's not undone.
- Drag "Beam 32nd sub" onto the 3rd note. Works. Drag "Beam middle" onto the 3rd node. Works. Undo. It's not undone.

Conclusion: "Beam start" and "No beam" have no effect. Undo has no effect.

R5367.