- GSNPositionedEditPartBase*: Avoid code duplication by pushing refreshDecorationFigure() to PositionedEditPartBase
- PositionedEditPartBase.refreshDecorationFigure()
  - Avoid SWT Image leak by reusing the same icons
  - Add more null-checks to implementation
- MarkerViewPart -> column updater: Avoid SWT Image leak by disposing old image before setting a new one