QAbstractProxyModel::sibling: treat row/column as offsets within the proxy

Qt5 allows QAIM subclasses to reimplement the sibling() method. Unfortunately,
the default QAbstractProxyModel's reimplementation differs in behavior to what
the Qt4 version was doing. In particular, the Qt4 version used to use the row
and column as positions within the proxy model, while the Qt5 version mistakenly
does this at the level of source model. This is arguably broken; the caller asks
for a sibling of the proxy index, not for a sibling within the proxy model.

This change makes the QAPM::sibling work explicitly in the same way as the Qt4
code behaved.

The reimplementation of QAbstractProxyModel::sibling was introduced in
9dfba89c28bbff3316cb7aed6c07f90c0f2d5a22. It was subsequently fixed with commit
999109866dbd350a29cc70815d0c772545c85746 not to return indexes from the source
model, but the logic was still different from the Qt4 version.

[ChangeLog][QtCore][QAbstractProxyModel] Fixed QAbstractProxyModel::sibling to
work in the same manner as the Qt4 code used to behave. Previously, Qt5's
implementation would treat the row and column as positions in the source model
instead of a position in the proxy itself.

Followup-to 9dfba89c28bbff3316cb7aed6c07f90c0f2d5a22 and
999109866dbd350a29cc70815d0c772545c85746
Change-Id: Ia25027b2ad9e4777ba28de2d2226d48f8cccf587
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>