This is the second part of 337a0aaa692b794167a713d4af25ebcc86f492fb, it
allows changing the sash width through the swt-sash-width CSS property.

Example usage in CSS:
.MPartSashContainer {
  swt-sash-width: 2px;
}

Change-Id: If358d750b783791aedc2d7b05686f0f89b3a6df6
Signed-off-by: Pierre-Yves Bigourdan <PyvesDev@gmail.com>