Add C++11 constexpr to 3D vector classes

This turns QVector2D(1,1) into compile-time expressions when using
a C++11 compiler.

Change-Id: Ifea2d1cdfd5481300f9a5a0c7bbe8bfe9115c636
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>