Fix Qt 5 todo issues for QSizePolicy.

Fix Qt 5 todo issues for QSizePolicy.

* Merge the two public ctors.
* Use bitflags instead of shifting bits (more readable).
* Add autotest
* Use int datatype for the "stretch setters". (values out of bounds are clamped)

Streaming to QDataStream will still use the Qt 4 format.

Task-number: QTBUG-25100

Change-Id: Iecb1e78cb12717e4d84448484c3ad8ca469d571a
Reviewed-by: Paul Olav Tvete <paul.tvete@nokia.com>