Reorganize examples #564

I think we should:

• remove all nb examples except for quickstart, test_gc, and for now imagewidget. Once we have imgui integration we can remove imagewidget as well and just have the quickstart notebook
• move examples/desktop to just examples/ and have these as the main examples for everything, docs, tests, users, etc. Move any examples that are currently only in the notebook to here.

@clewis7 thoughts? I think this will make everything easier to maintain, and easier for users too since there would only be one set of examples.