Form_Field_Readonly: more value_list fixes

Slightly modified this part. Now it works with model based fields (like DropDown with associated model) *and* also fields without associated model like the ones defined as `$model->field->listData(array(0=>'Person',1=>'Company'));`
Merge pull request #429 from DarkSide666/patch-3

Form_Field_Readonly: more value_list fixes