Update existing material on import instead of adding new one

When receiving a material from an external source such as Designer ("Send To Stager"), if the material already exists in Stager (same ID), please update it instead of creating a new one, and update the scene accordingly with the updated material.

What is currently happening is a new material is being added to the library which creates the following problems:

- we quickly end up with many similar materials into the library

- we have to re-assign the new material to the objects in the scene and re-configure them.

You may make this behavior configurable if you wish, but at least provide the option to update instead of creating new. This makes the most sense I think if the material has the same ID, then it's meant to replace the existing one.