Scene performance optimization

I’ve done extensive performance profiling and optimization of Slicer’s scene management. For large atlas scenes loading/import speed is about 2x faster, scene closing is about 10-20x faster.

Typical processing times BrainAtlas2012.mrb:
•	Load: 39.0 sec => 25.2 sec (1.5x)
•	Import (in addition to one atlas already in the scene): 91.9 sec => 32.1 sec (2.9x)
•	Scene close: 21.4 sec => 1.1 sec (19x)

This CTK modification was also applied to obtain the above results: https://github.com/commontk/CTK/pull/378

Each commit in this branch can be reviewed and merged independently.