[451852] Introduce visual (V) generic parameter to IVisualPart.
[451852] Introduce visual (V) parameter to AbstractVisualPart#doRefreshVisuals().
[NONE] Refactor default handle parts.

- Merged FXConnectionSegmentHandlePart and FXSegmentHandlePart, renamed into FXCircleSegmentHandlePart.
- Renamed FXCornerHandlePart into FXRectangleCornerHandlePart.
[NONE] Fix NPE in FXGeometricCurvePart.
[NONE] Made visual creation and registration more mature.

Ensured that visuals can be registered lazily, and that registration is performed whenever a viewer link is available.