Blender Git Loki

UI: Add Free Handle Types to CurveProfile Widget

Under the hood the CurveProfile widget (used for bevel custom profiles)
uses a bezier curve, but right now though it only supports two of the
bezier curve handle types, vector and auto. This patch adds support for
free handles and adds all of the logic for editing them.

This is the first step to the ability to import and export curve objects
in the widget.

There's some code cleanup in curveprofile.c. Movement for handles and
control points is abstracted to functions there rather than happening
in interface_handlers.c.

An "Apply Preset" button is also added, which solves a confusing issue
where you apply a preset, then change the number of samples and the
preset doesn't change. The button makes it clear that the preset needs
to be reapplied.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D6470