Cleanup visualization manager pause APIs.

Make the IsUpdating flag setter internal and add two methods to the
interface Pause() and UnPause(). Use these new methods to defer
visualization updates during model loading. Also adds an check whether
visualization manager is paused to ignore node property updates.
Add two interfaces.

ICleanup interface to be used on objects requiring shutdown cleanup.
IVisualizationManager interface moved from its previous location in
VisualizationManager.
Remove IVisualizationManager from VisualizationManager.
Fix backwards pause and unpause on drag selection.
Initialize cachedMirrorData with a null.
Add the previously created interfaces to the project.
Don't create null MirrorData objects.

Standardize on return null for CachedValue.
Make the print value method check for null CachedValue.