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

[kwokcb]

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)