Graph Editor: Functional graphs in loaded documents are editable

If a document is loaded and it contains a definition using functional nodegraphs, these graphs show
up as being editable. This could cause problems if edited as the instances could no longer work.
If you select an instance and look at it's graph, it is also editable.

Query as to what the desired behaviour is here ?

Perhaps make these non-editable (when examining the graph via the instance and
also the graph itself) for now to keep things "safe" ?

See this [test](https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/resources/Materials/TestSuite/stdlib/shader/surface.mtlx) taken from test suite. The checker definition's nodegraph is editable.

1. Graph as loaded. Functional graph is show and is editable.
2. Expanding an instance, gives it's nodegraph, which is also editable.

Reproduce the behavior in the Graph Editor using resources/Materials/TestSuite/stdlib/shader/surface.mtlx, focusing on the checker definition and an expanded instance. First determine the intended behavior for functional nodegraphs, then verify the chosen behavior for both the loaded graph and the instance graph; the issue does not name implementation files or tests.

Written by the indexing model from the issue text.