When creating a new foreign key constraint in the Edit Table dialog, a
list of tables to reference is displayed. This list is updated whenever
the name of the edited table changes because this is the only table the
name of which can change while the Edit Table dialog is opened. However,
this meant that when the name of the table changes to some existing
table name, the field list of that existing table is replaced by the
field list of the current table. If the name of the current table is
changed once again, it is deleted entirely from the list. This commit
fixes this behaviour by separating the static part of the table and
field list from the one table which can change.
See issue #1991.
