Display control points

[uliska]

When the mouse hovers over a bezier curve or a curve is selected we should display control points, i.e. add SVG elements to draw them.
This will be a significant improvement over the respective Layout Control Option (that requires recompilation).

Additional wishes:

• do that also when a note is selected (hovered) where a curve is attached to
• do that for all curves of a document (using a UI toggle)

[PeterBjuhr]

I've had a clear idea for this all along: I think we should add two small circles to the SVG file, when moving them the curve or shape is adjusted. This should also give the control points for the compiled edits. We can only hope that the transition from SVG coordinates to LilyPond coordinates is as easy as with the offsets.

Preferably the circles only appears when the curve is selected for editing.

[uliska]

The circles (or crosses) should also be connected to the Prüfung with Lines. Just as what the respective layout Controlling mode does.

Yes, These should appear automatically upon aelection Pr hovering, ideally also when the notehead is selected where the curve is attached to.