Graph Editor: Handle Nodegraph Implementations in Non-Library files.

## Issue

- Currently nodegraph implementations which are embedded in non-ibrary files which are loaded in are
viewable and editable.
- The definition does not show up as an option for adding a new node.

## Suggestion
- Either hide or template (make read-only) these graphs.
- Add the definition to the "add" UI. This should be discussed as to what it means if the file is unloaded -- should the definition stay or not. Suggestion is to mark and clear these definitions on a new file load / file clear.

### Example:
Load in the OpenPBR combined file provided in OpenPBR slack (included here) into editor.
The nodegraph implementation (circled) loads in and is editable.

[open_pbr_surface_combined - .txt]
(https://github.com/AcademySoftwareFoundation/MaterialX/files/13951690/open_pbr_surface_combined.-.txt)

Load the attached open_pbr_surface_combined .txt in the Graph Editor and inspect the file-loading, embedded nodegraph, and add-node UI paths. Confirm the intended behavior for definitions from non-library files, including whether they are editable or read-only and what happens when the file is unloaded or cleared.

Written by the indexing model from the issue text.