Add a loading state to topology when generating larger models

[andrew-ronaldson]

Describe the problem
There was a recent request to add a loading phase while topology visualizations are being rendered. Typically, the rendering is fairly quick on my testing but if a user is rendering 1000s of nodes or complex layouts it would be beneficial to show a state of transition so the user can expect a result is coming.

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Jira Issue: PF-2005