Harmonize GridLayout2 and GridData2 with SWT classes

Similar to https://github.com/eclipse-windowbuilder/windowbuilder/issues/963, the [GridLayout2](https://github.com/eclipse-windowbuilder/windowbuilder/blob/master/org.eclipse.wb.rcp/src/org/eclipse/wb/internal/rcp/model/layout/grid/GridLayout2.java) and [GridData2](https://github.com/eclipse-windowbuilder/windowbuilder/blob/master/org.eclipse.wb.rcp/src/org/eclipse/wb/internal/rcp/model/layout/grid/GridData2.java) are old copies of the SWT layouts, which store additional information that are relevant during design time.

We should separate the designer information from the layout algorithm to make sure that our implementation matches the actual implementation.

Start with org.eclipse.wb.rcp/src/org/eclipse/wb/internal/rcp/model/layout/grid/GridLayout2.java and GridData2.java, then compare the related work in issue 963 and the corresponding SWT classes. The change is complete when designer-specific information is separated from the layout algorithm and the implementation matches SWT behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.