[DMN Designer] Allowed Values just for built in types

Citing form spec:

ItemDefinition element SHALL be defined using only one of the alternative ways:
- reference to a built-in or imported typeRef, possibly restricted with allowedValues;
- composition of ItemDefinition elements.

Because of the above, I think we should hide constraint component for ItemDefinitions of type structure.

Manual Acceptance test

• User is not able to add constraint (allowedValues) for item definition of Structure type
  • Create form scratch
  • Change existing that already had constraints