Add a inline QVector::append(const QVector &) overload

This is present in QList already and its lack is a nuisance when
switching from QList based code to QVector (which makes sense e.g.
if the item size exceeds sizeof(void*))

Also, albeit operator+=() and operator<<() exist, some people
simply prefer functions with real function names.

[ChangeLog][QtCore][QVector] Added QVector::append(const QVector &) overload

Change-Id: I9aae8223b086765625f2f3071fab5da0780f8a43
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>