Allow to initialize CascadeDropdown values

[ibelar]

Two values for proper initialization of the DropdownCascade is need: The actual value of the dropdown and the reference value for populating the dropdown items.

Usage:

$form = Form::addTo($app);

$form->addControl('cat', [Form\Control\Dropdown::class, 'model' => new Category($app->db)])->set(2);

$form->addControl('sub', [Form\Control\DropdownCascade::class, 'cascadeFrom' => 'cat', 'reference' => Category::hinting()->fieldName()->SubCategories])->set(9);

$form->addControl('prod', [Form\Control\DropdownCascade::class, 'cascadeFrom' => 'sub', 'reference' => SubCategory::hinting()->fieldName()->Products])->set(3)

[mvorisek]

If the 'category_id' and 'sub_category_id' is from some demo/doc, replace it with SubCategory::hinting()->fieldName()->...

Also check the default values in Behat test.