Reimplement QSqlQueryModel::setQuery()

Current behaviour is to remove all old rows and then insert the new rows. A different and better( ? ) approach is to compare the query size, add or remove rows, and then emit dataChanged for the entire model. For databases that doesn't report the query size we'll have the prefetch size.

Check all open QSqlQueryModel and QSqlTableModel tasks to see if other improvements can be done at the same time.