Second draft for a toManyTarget setter, which is implemented in
CayenneDataObject (Davids intention). In difference to addToManyTarget
or removeFromManyTarget this method takes a input Collection<? extends
DataObject> and a boolean delete parameter for a additionally deletion
of DataObject, which relationships were removed.

The method documentation should explain exactly what this method does.

The superclass.vm has 2 new methods for accessing the setToManyTarget
with the relationName and Collection. The method without deletion flag
calls setToManyTarget with deletion=false. Second method takes deletion
flag.