[DMN Editor] Add support to define data-types

As a practitioner, I want to be able to define Data Types in the UI, so that when the DMN engine evaluates a decision table input/output it will check if the type of the value matches the specified type.

Verification conditions:
Support added for the User to define data-types (ItemDefinition).

This may become a top-level task when I get to work on it; as the requirement is quite large and encompasses nested data-types (a whole new grid will be required); and definition of enums, constrains on values etc.