Moves this one row before where it already is (synchronizes the values of the parameters in this with the values at the new row).
If the iterator was on the data model's first row, then it can't be moved backwards anymore, and the returned value is
also that the "current-row" property is set to -1 (which means that
is_valid would return
This function can return
false if it was not allowed to be moved (as it emits the "validate-set" signal before being moved).
When this function returns
true, then this has actually been moved to the next row, but some
values may not have been read correctly in the row, in which case the correcsponding Holder
will be left invalid.
a DataModelIter object