Cast to IList<IDiagramElement> when return in GetDrawableObject().
Core/EasyDiagramEditorBase.cs
Delete serializable object from RootObjectBase class.
Revert "Delete serializable object from RootObjectBase class."

This reverts commit 4bb7eb25e3404243da804bea3a58603c7798b0ae.
Delete serializable attribute from RootObjectBase.
Add function of initalize root object.
Create interface of set/get method of view rect.