Ensure "InitializeForActiveDocumentOnIdle" is called from idle thread
Fixed a crashing issue when DynamoModel is accessed too early
Introduced a new "NotifyRenderPackagesReadyAsyncTask" async task
Introduced virtual "VisualizationManager.HandleRenderPackagesReadyCore"
Updated RequestNodeVisualUpdate to use NotifyRenderPackagesReadyAsyncTask
Merge pull request #2793 from Benglin/RC0.7.3_Revit2015

Cross merging code changes into release branches