The final PR for the external module feature that allows materials to
be stored in an external datastore, webservice, etc.
This includes the final material manager classes, and the UI support in
the form of commands and preference pages.
* Materials: Model Manager External Interface
Implement the ModelManagerExternal class for the external Materials
interface.
This is part of the ongoing merges of code to support external material
interfaces. In this PR the ModelManagerExternal class is implemented,
along with changes to supporting classes required for this class.
* Apply reviewer feedback
The ExternalManager class calls python functions in an external module
to create, read, update, and delete material definitions. The API
provided by the modules must conform to that defined in the
MaterialManagerExternal.py file.
All communications with the external module is routed through this
class.
