Add IFormField::getFieldHtml()

[Gravatronics]

IFormField::getHtml() returns the form field with its surrounding structure (dl + children). To support more specializes form field containers, just having a method that return all required HTML code for the form field (including JS-Code) without the surrounding structure is required.

This new method will be IFormField::getFieldHtml(). The (new) template used by IFormField::getHtml() can than simply call {@$field->getFieldHtml()}.