Allow setting a form-element prefix.

For me this was useful in order to use the chooser on cms_page/edit

There might be a cleaner solution with extending from a different, more general class.
But at least this solution is simple and does not break anything.

Example:

        $thumbnailField = $fieldset->addField('thumbnail', 'text', array(
            'label' => Mage::helper('cms')->__('Thumbnail'),
            'title' => Mage::helper('cms')->__('Thumbnail'),
            'disabled' => false,
            'value' => $model->getThumbnail(),
        ));

        $renderer = Mage::app()->getLayout()->createBlock('widgetimagechooser/chooser');
        $renderer->setElementPrefix('page_');
        $thumbnailField->setRenderer($renderer);