(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>