* fixed ROIUtil.getMassCenter() method.
* modified ROI2DPoint rendering.
* fixed vertical and horizontal ROI2DLine selection.
* added ROI contour size field (only for drawing).
* loader dialog show image preview even for "all files" filter.
* added GeneralPreferences.get/setLoaderFolder(..) to retrieve the
current "Open" folder.
* do not clear (remove ROI and overlay) sequence object on close (GC
should do it anyway)
* changed way of closing opened windows on exit.
* fixed histogram refresh.
* fixed InstanceProcessors.submit(..) method.
* tweaks / fixes on GUI refresh.
* refactoring.
* fixed shared LUT on viewer duplication (regression).
* Postponed VTK object creation in ROI2DShape.