Add new buttons to zoom closer to the data

Two new buttons 'ResetCameraClose' and 'ZoomCloserToData' has been added at the right of the existing "Reset Camera" and "Zoom To Data" buttons. These buttons reset the camera to maximize the occupation, in the screen, of the whole scene bounding box (ResetCameraClose) or of the active source bounding box (ZoomCloseToData).

It uses the ResetCameraScreenSpace method defined in vtkRenderer