Ensure "InitializeForActiveDocumentOnIdle" is called from idle thread

Conflicts:
	src/DynamoRevit/Models/RevitDynamoModel.cs
	src/Libraries/Revit/RevitServices/Materials/Materials.cs
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
Removed Revit 2015 specific code from "InitializeMaterials" method