Harmonize MigLayout specification composites

The [CellSpecificationComposite](https://github.com/eclipse-windowbuilder/windowbuilder/blob/master/org.eclipse.wb.swing.MigLayout/src/org/eclipse/wb/internal/swing/MigLayout/model/ui/CellSpecificationComposite.java) and [DimensionSpecificationComposite](https://github.com/eclipse-windowbuilder/windowbuilder/blob/master/org.eclipse.wb.swing.MigLayout/src/org/eclipse/wb/internal/swing/MigLayout/model/ui/DimensionSpecificationComposite.java) are functionally identical. We should extract the common logging into an abstract base class, then the actual implementations only need to implement a ´setString´ and ´getString´ method.

Start by comparing CellSpecificationComposite.java and DimensionSpecificationComposite.java, focusing on their duplicated logging and string-handling behavior. Extract the common logging into an abstract base class, leaving each concrete composite to provide setString and getString; done means both implementations use the shared behavior without changing their functional results.

Written by the indexing model from the issue text.