Disconnect slots from the old sourcemodel in QIdentityProxyModel

Disconnect slots from the old sourcemodel in QIdentityProxyModel

When setting a new sourcemodel QIdentityProxyModel::setSourceModel
tries to disconnect from signals belonging to the NEW model instead of
from the existing sourceModel(). QIdentityProxyModel thus receives
signals from both the old model(s) and the new model. This results in
ASSERTS triggering in various slots, for example:
"Q_ASSERT(topLeft.isValid() ? topLeft.model() == model : true);"
in QIdentityProxyModelPrivate::_q_sourceDataChanged.

Change-Id: Ic6f65a9ee10981d00206335f2edef78272fadc1a
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>