Great progress here, xie-xie!  :) Did a bit of editing and cherry-picking
of the submission, stripping only typos and unnecessary line noise.

Submitted by: Liu, 周

Using helpers.toList('parent.tag') makes it explicit that we expect an item to be a list and solves issues with lists containing only one item.

this commit adds two new helper functions to use uuid information from the config.xml.

1) get a tagname by uuid, or __not_found__ if uuid is not in the config.xml
{{ helpers.getUUIDtag('a8da740a-046a-4994-8535-89f5fff47393') }}

2) returns an object by uuid, ready to use
{{ helpers.getUUID('a8da740a-046a-4994-8535-89f5fff47393').number }}

(mvc model) add ModelRelationField type, creates a optionlist populated by array type items from other entities.

Example definition from trafficshaper model, relating a pipe to a rule.

                <target type="ModelRelationField">
                    <Model>
                        <pipes>
                            <source>OPNsense.TrafficShaper.TrafficShaper</source>
                            <items>pipes.pipe</items>
                            <display>description</display>
                        </pipes>
                    </Model>
                </target>