Qto_SlabBaseQuantities: Rename Width to Thickness to avoid confusion with Depth

In Qto_SlabBaseQuantities, Width denotes the slab's thickness (normal to its surface), while Depth denotes a footprint dimension — the opposite of common usage. The spec itself acknowledges this by warning not to confuse the two. **Renaming Width → Thickness** (consistent with e.g. "TotalThickness" of IfcMaterialLayerSet) would eliminate the ambiguity without breaking the naming logic of Length / Depth for footprint dimensions.

Start by locating the Qto_SlabBaseQuantities definition in the IFC4.x specification and review how Width, Thickness, Length, and Depth are described. Check the related references and wording for consistency, then confirm the renamed property removes the ambiguity without changing the footprint-dimension naming logic.

Written by the indexing model from the issue text.