Adding support for 'stacks' for nationality (and other properties)

[timgdavies]

The visual language has the concept of 'stacks' to allow display of multiple properties of an entity, person or relationship.

At present, we only have overlays for nationality/jursidiction (?) (using flags), but these do not display as stacks when multiple are present.

For example, Person 1 below is both Ukranian and British nationality, but only the first of these is shown on the diagram.

"personType": "knownPerson",
        "names": [
            {
                "type": "individual",
                "fullName": "Person 1"
            }
        ],
        "nationalities": [
            {
                "name": "Ukrainian",
                "code": "UA"
            },
            {
                "name": "British",
                "code": "GB"
            }
        ]

[kd-ods]

This is somewhat related to #55. There's a choice to be made for each (relevant) property about whether its value is represented diagrammatically or by the mechanism in #55.