API for producing scene graph for model elements

[philip-alldredge]

Closes #2778

[philip-alldredge]

@lwrage this pull request adds some APIs that would provide plugins with additional visualization capabilities. The code is fairly small but it isn't used within the editor itself. However, it does include a basic SWTBot test to ensure that the API doesn't throw and it produces the expected scene graph nodes.

[philip-alldredge]

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- Philip Alldredge