Added SpecTableLayout

That's it.

If you have any remarks about following Spec conventions, or if something's missing, let me know.

Also I have to note code duplication in `#subwidget:fractions:offsets:` and `#subwidget:layoutFrame:` methods of `SpecLayoutAdd`, and now also in `#subwidget:` method in `SpecTableLayoutAdd`. I don't know what you do in Smalltalk in such situations (create a trait?), but common code needs to be extracted.
I think it would be better to implement a message like #isContainer on ContainerModel and ComposableModel.
I tend to avoid as much as possible usages of #isKindOf