Ensure shared_null(s) are statically initialized on VS 2010

Ensure shared_null(s) are statically initialized on VS 2010

This removes const qualification on data members of QConst*Data, which
was subjecting QString's and QByteArray's shared_null to the "order of
static initialization fiasco", with up-to-date VS 2010.

Furthermore, the const qualification in the places where it was removed
had little meaning and no value. It was unnecessary. As such, "Const"
was removed from the struct's names and "Static" used in its place, to
imply their usefulness in supporting statically-initialized fixed-size
(string and byte) containers.

A test case was added to QArrayData as that is meant to replace both
QStringData and QByteArrayData in the near future.

VS issue reported at:
https://connect.microsoft.com/VisualStudio/feedback/details/716461

Change-Id: I3d86f2a387a68f359bb3d8f4d10cf3da51c6ecf7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>