Slur height adjustment

Slurs have four control points. When I want to change the height of a slur I have to independently adjust the two middle points by the same amount to change the curvature on the slur. Moving a single inner point alters the symmetry of the slur. This makes it difficult to (1) preserve the same symmetry of a slur when adjusting its height, and (2) visually decide on the best curvature of a slur since the shape of the slur needs to be adjusted with two separate point movements.

It would be very useful to be able to control the height (overall curvature) of a slur while preserving its symmetry (or lack of it). A control point at the midpoint is one solution, or being able to select both inner control points at the same time.

Attached are suggested improvements in the slur typesetting for the main demo song. (slurs should not start/stop on a staff line for better readability, should be symmetric in a majority of cases, and not have more curvature than is necessary).