The dtd file is very old and doesn't match gtk+.xml from glade gtk plugin.
Update it so that the future additions pass verification.
Add add-child-verify-function, add-child-function, remove-child-function,
get-property-function, verify-function, special-child-type and
packing-actions to glade-widget-class element definition.
Additionally define add-child-verify-function,
child-action-activate-function, get-property-function, special-child-type,
and packing-actions elements.
For packing-action, define action element and attributes.
Define and add displayable-values to property definition.
Remove parameter-spec's duplicate "paremter-spec" and add min child,
along with its definition.