QStandardItemModel::setItemData replaces the content of an item data
with the new values rather than updating/inserting which is
the behavior for QAbstractItemModel. This patch aims to unify the
behavior.
[ChangeLog][QtWidgets][QStandardItemModel] Fixed setItemData()
incorrectly deleting unmodified data. That behavior is not
following QAbstractItemModel's documented behavior which is
no modification of data not provided in parameter.
Task-number: QTBUG-45114
Task-number: QTBUG-10872
Change-Id: I2be40cee372b68d9f71c976548ecda6dc3011241
Reviewed-by: David Faure <david.faure@kdab.com>