Feature request: Display bezier control-points

[Urs Liska]

Sorry if I should be asking for something that is already possible, but 
at least I don't know of it ...

Tweaking slurs and similar curves is a matter of trial and error, 
although David's functions that expect offsets instead of hardcoded 
control-points greatly simplify it.

It would be a really valuable feature if one could let LilyPond display 
the control-points of a bezier curve.
I would imagine something like a cross or a point at the middle two 
control-points.

It should be settable through e.g.
    \paper { display-control-points = ##t }
(like annotate-spacing)

Is that of general interest?
And does it look complicated?

Best
Urs