When an artifact link is created or modified, let one plugin provide a custom type (aka nature) for the artefact link.

An artefact link may be eligible for multiple types: child type (hierarchy) has priority, then custom type provided by plugin, then existing type. This implies a custom type will always override the user choice (for now).

This change introduces the new hook TRACKER_EVENT_ARTIFACT_LINK_NATURE_REQUESTED in the Tracker plugin so that other plugins can test link parameters and return a custom type.