Convert more QVector*D member functions to constexpr

constexpr requires inline functions. We convert more member functions
to inline functions while retaining a version of those functions
inside QtGui. This prevents any BC breakage.

Change-Id: I325b78da6b41611bda3994869f474c700b6fb306
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>