KERML — Add derived property for targetFeature

When a Feature has chainingFeatures, then it is sometimes useful to reference that last Feature in the chain in situations in which one would otherwise reference the owning Feature itself if there was no chain. For instance, in a graphical view, an edge with a chained Feature at an end will generally point to the last chainedFeature (possibly nested) rather than the chained Feature itself.

Currently, this means that it is necessary to check for a feature f whether f.chainingFeature is non-empty and, if so, use f.chainingFeature->last() instead of f. It would be more convenient to have a derived property of Feature (called, say, targetFeature) that would do this automatically, that is, be derived as

targetFeature = if chainingFeature.isEmpty() then self else chainingFeature->last() end if