This is only a preview, I want to get some feedback about the UI:

* Is the placement of the curve in sculpt-properties and editbuttons-brush OK?

* Is the Default-curve OK? It differs a little bit from old behaviour. I Need
advise from sculpt-pros. If you find the perfect curve, give me the blend-file,
I can integrate it.

* As it is now, only one curve is used. Do we need seperate curves for each
brush (paint, smooth, flatten, inflate....) ? And if so, what UI? inkable
curve-data)

* The curve-api is not very generalized. I added a "default"-button to reset
the curve. The tools-menu of the curve-button provides "reset Curve",
but this yields a straight line.