Include referenced acts/facts/duties in TreeView in flinteditor

[pimotte]

Currently, the TreeView of the FLINT-editor lists the acts, facts, and duties. This is kind of barebones and should be improved in the following way:

• The toplevel acts/facts/duties should be able to be unfolded, and the children should be the acts/facts/duties that are referenced in the properties of the acts/facts/duties.

For example, if this act is unfolded in the TreeView:

        {
            "act": "<<voorafgaand controleren>>",
            "actor": "[RVO]",
            "action": "[controleren]",
            "object": "[betalingsaanvraag]",
            "interested-party": "[agrarisch collectief]",
            "preconditions": {
                "expression": "AND",
                "operands": [
                    "[betalingsaanvraag]",
                    "[goedgekeurde gebiedsaanvraag]"
                ]
            },
            "create": [
                "<terugleggen geconstateerde fouten>"
            ],
            "terminate": [
                "[voorafgaande controles]"
            ]
        }

The children shown should be [RVO] [betalingsaanvraag] [agrarisch collectief] [goedgekeurde gebiedsaanvraag] <terugleggen geconstateerde fouten> [voorafgaande controles]

Clicking the child should take the user to the definition of that child.
The children should be able to be unfolded in the same way to show their references.
Duplicate children should only appear once.

TODO:

• Figure out how the law-reg ModelValidator API should be extended to support this usecase
• Extend law-reg in the way needed
• Update the flinteditor to make use of the new/extended API