Move CasexTraits from qstring.cpp to qunicodetables_p.h

and implement TitlecaseTraits as well,
to be re-used in the following commit.

Change-Id: I5c0bff42fe3b9bc9c2454cc16916cc2be87f604f
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>