Represents a window featuring a specification viewer and
a geometry viewer.
The specification viewer is located in the left part of the window
and displays the document's specification tree.
The geometry viewer is located in the right part of the window and
displays the document's geometry, and can thus
be a Viewer2D or a Viewer3D, according to the document type.
Even if generally the two viewers are simultaneously displayed,
one viewer or the other can be hidden thanks to the
CatSpecsAndGeomWindowLayout enumeration.