Added cubic bezier splines to transfer function graph

Added cubic bezier splines to the transfer function graph. It is implemented via double clicking a control point which then shows two smaller green points which I called nodes. They influence the curvature of the line on the right side of the control point and are restricted to move only in between itself and right control point.

After a double click the nodes can be moved similarly as the control points via click and drag. It has a guiding line showing which control point they belong to.

By shift-clicking a node you combine them and move both points at the same time, essentially making it a quadratic bezier spline instead.

Biggest issue is that the nodes are by default set to the same position as the control point and can be difficult to interact with at times