[XmlEditor] Checking IdeApp.Workspace != null & Listening for FileAdded/RemovedToProject

[XmlEditor] Checking IdeApp.Workspace != null before accessing it in UpdateOwnerProjects
[XmlEditor] Listening to FileAddedToProject/FileRemovedFromProject and updating OwnerProjects

Didn't know IdeApp.Workspace is not always set(at least in AspMvc unit tests is not)
Forgot to handle IdeApp.Workspace.FileAddedToProject and IdeApp.Workspace.FileRemovedFromProject when was coping functionality from C# implementaiton...