Need efficient property-propagation mechanism

Occasionally a child element depends on a property that should be
propagated down from a parent, rather than being e.g. global.
In these cases the child can walk the parent-hierarchy to find
the value of the property, but this is often expensive, sometimes
prohibitively so.

Thus it would be helpful if there were e.g. a property-modifying
keyword that made the property work the same way that e.g. the
Item's "visible" property does. I.e. explicit changes to the property
should be propagated to any children with the same property.