- UBGraphicsScene calls UBGraphicsStroke::addPoint, which returns a list
of points that can be drawn. It may be none (e.g we discard very small
segments), one (if we do no interpolation) or several.

- Added a UBInterpolator base, abstract class. Various interpolation
methods can be added easily.

- Current methods: Basic spline (custom), Catmull-Rom spline (based on
alglib), and Bézier

- Added a setting to toggle interpolation. Added this to the UI as well