add new event to rewrite frontend filter widget

[zonky2]

add new event to rewrite frontend filter widget e.g. to add a HTML5 placeholder, mandatory flag or own validation

[zonky2]

see #1099

[zonky2]

and see MetaModels/attribute_select#54

[zonky2]

currently we can use the Contao hook parseFrontendTemplate or adapt the template or you can work with GetAttributesFromDcaEvent before teh widget ist rendered - see

core/src/Filter/Setting/Simple.php

Line 457 in c790a5f

$event = new GetAttributesFromDcaEvent(

e.g. at text widget

        $eval                = $event->getResult();
        $eval['placeholder'] = 'Moin';
        $event->setResult($eval);