label hierarchy, display

in the database of labels, keep relationships e.g. "isa", (1:many , like traits or multiply inheritance),
display these in the label list, e.g.
if you label some traffic with "car", "van", "hatchback car", "sedan", the list would show:-

vehicle:
truck:
articulated lorry
van
bus:
bus
school bus
double decker bus
car:
car
hatchback car
sedan
pickup truck
sports car
taxi
minibus

more specific labels like 'hatchback car' would still show up as 'car' labels. Allow being as specific as possible with the labelling, whilst still providing broadly useful data.

The issue names no files, tests, or entry points. Start by locating the label database and the label-list rendering code, then determine how hierarchical and multiple-parent relationships should be represented and displayed. Done means specific labels remain selectable while the list presents their broader parent labels and relationships clearly.

Written by the indexing model from the issue text.