ENH: Minor performance improvement of Slicer startup
ENH: vtkEventBroker performnce improvement

Use a set instead of a vector to allow faster finding and deletion of observations
ENH: Optimized performance of observations management in vtkMRMLScene

Use a std::multimap instead of two separate ReferencedIDs and ReferencingNodes vectors to make lookup, insertion, and deletion much faster (especially scene close takes much shorter time now)
ENH: Optimized performance of qMRMLSceneModel

qMRMLSceneModel::nodeIndex is called tens of millions of times, therefore it's optimum performance is critical.
Find MRML nodes by object pointer instead of ID comparison to improve the performance.
ENH: Tuned node observation rule in qMRMLSortFilterProxyModel

The previous behavior was probably mostly OK, but the current implementation seems to be correct.