Form_Field: cleanup

1) Form_Field_Text setFieldHint same as Form_Field
2) Removed obsolete methods setFieldTitle, clearFieldValue, setDefault, getDefault. These are obsolete since 4.1 and needs to be removed at some point. Also I checked that ATK4, atk4-addons and ambient-addons don't use these.
3) $mandatory_template variable fix
4) some comments fixed
5) Form_Button - removed unused variables