Enable constexpr for basic geometry types (QPoint, QSize,...)

A lot of geometry calculations and verifications can be done at compile time.

It would be convenient to use the same types in such compile-time code as in runtime geometry calculations. Therefore, it would be useful to be able to create constexpr versions of

• QPoint / QPointF
• QSize /QSizeF
• QRect /QRectF
• QLine / QLineF
• if feasible also QPolygon / QPolygonF

At this time, I am not aware of any (current or upcoming) std types that could replace the above mentioned types in a constexpr-enabled way.