ENH: Added SubjectHierarchyItemResolvedEvent to get notified about imported/restored items

Although it was already possible to get notifications of most subject hierarchy item events, it was not possible to add observations for adding items when importing a scene or restoring a scene view. Now, SubjectHierarchyItemResolvedEvent is invoked when a scene is being imported/restored (i.e. resolving unresolved items), allowing the developer to get notified about such events, similarly to how they can get notified about imported/restored MRML nodes.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25904 3bd1e089-480b-0410-8dfb-8563597acbee