This commit adds a "soft" selection to curve control points,
a sculpt tool to paint the selection, and uses the selection influence
in the existing sculpt brushes.
The selection is the inverse of the "mask" from mesh sculpt mode currently.
That change is described in more detail here: https://devtalk.blender.org/t/paint-mode-design-discussion-feedback/24243
Since some sculpt tools are really "per curve" tools, they use the average point selection
of all of their points. The delete brush considers a curve selected if any of its points
have a non-zero selection.
The main limitation currently is that we can't see the values in the viewport at all.