ENH: Simplified subject hierarchy node resolving process

Subject hierarchy node resolving happens when a second subject hierarchy node appears in the scene - usually when a scene is imported. Then merging of the new SH nodes into the first one happens, after which the new (and now empty) SH nodes are removed. It has been possible to observe items added when resolving using SubjectHierarchyItemResolvedEvent, but it was too complicated, as Add and Resolve basically mean the same operation, just in different states. Now SubjectHierarchyItemAddedEvent is invoked as usual when resolving, but it is now possible to get notified about resolving operations using SubjectHierarchyStartResolveEvent and SubjectHierarchyEndResolveEvent. This also allows processing after scene import without using a timer.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26063 3bd1e089-480b-0410-8dfb-8563597acbee