Form_Field_Readonly: now works nicely with ValueList based fields

This is cool ! :)
Now readonly form fields at last nicely can replace ValueList based form fields like DropDown, Radio, CheckboxList. Readonly field will not show ID anymore, but get and show related model title field value (with just one simple SQL request).
Merge pull request #423 from DarkSide666/patch-4

Form_Field_Readonly: now works nicely with ValueList based fields