use field type select if a field has an foreign key

[zukunft]

If a field is a reference to another table, the default parameters for this field should be a selection. This way for example these lines would not be needed any more in the mask code

    $this->fields->subtable_id
        ->setLabel("Please select")
        ->setType("select")
        ->setSource(P4A::singleton()->subtable)
        ->setSourceDescriptionField("description");

For the default description field probably the best is to use the first non key field in the subtable. Additional the default setting should be to include a NULL key with the description "not set".