Convenience to avoid annoying detaching (instead of using at()),
especially on temporary vectors (returned by functions or so).
[ChangeLog][QtCore][QVector] Added the convenience constFirst
and constLast functions.
Change-Id: If61b1f0096f6a7a1c9074340e237cc2376ce3d18
Task-number: QTBUG-46026
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>