Remove code duplication in GridData2 #964

Rather than copying the fields of the SWT `GridData`, the `GridData2`
now stores a reference of the original `GridData` object.

Using a copy of the original class brings the risk of incompatibilities,
if the class is ever updated on the SWT side. So its only purpose should
be to cache the bounds for the grid cell and delegate all other task to
the base class.

Contributes to
#964

Author: ptziegler