Fix tag data not being updated by a TagModifyJob

TagModelPrivate::monitoredTagChanged() is called as expected,
but in the case where the tag is not being reparented (the
usual case), the new tag is not copied back into the parent's
mChildTags list.  This means that TagModel::data() still accesses
the old tag via tagForIndex() and hence does not reflect the
change.

REVIEW:118103