Missed file in last commit
Improves the support for chained widgets

JComboboxes or JTextFields which value depend ('is chained to') other
components are handled by AbstractForm
Implements ChainedHandler in AbstractSubForm
Fixes how the foreign keys of chained widget is matched

When there are more than one foreign key we should not only check
that all the values of a foreign key are contained, but also they
have the same order
Clean up