Method to get the form control. This string serves as a container for all form fields. For example, if there is a field named 'foo' and a field named 'bar' and the form control is empty the fields will be rendered like: <input name="foo" /> and <input name="bar" />. If the form control is set to 'joomla' however, the fields would be rendered like: <input name="joomla[foo]" /> and <input name="joomla[bar]" />.