This commit adds an option to interpolate the number of control points in new
curves based on the count in neighboring existing curves. The idea is to provide
a more automatic default than manually controlling the number of points in a
curve, so users don't have to think about the resolution quite as much.

Internally, some utilities for creating new curves are extracted to a new header
file. These can be used for the various nodes and operators that create new curves.

The top-bar UI will be adjusted in a separate patch, probably moving all of
the settings that affect the size and shape of the new curves into a popover.