First stage of RM#3180: allowing bridges to be expanded into its shadow assets, so image varieties can be displayed - bringing it into line with the backend asset map.
They cannot be selected yet, because these assets are usually defined as "dependant" which is blocked from selection.
In lieu of finding a better way of showing an expandable bridge with an unknown number of parents, they are shown in the GUI as expandable with "?" children (would be shown as "N/A" in the backend).