Use a QVector<int> instead of a QSet<int> in itemviews/models.

Use a QVector<int> instead of a QSet<int> in itemviews/models.

The QSet<int> is a more expensive container to use and create, so
it should be avoided.

This is source incompatible compared to earlier Qt 5 for
QAbstractItemView subclasses which reimplement dataChanged, but this
patch changes nothing compared to already-present SiC compared
to Qt 4.

Change-Id: Id95391dfd62a0a7f487a8765790b007badefb937
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>