Create example application with backend

Edit from @afonsobspinto:

We want this application to work as somewhat real documentation / define best practices to develop new applications that want to use geppetto backend.
It should be maintained with the latest.
It should cover parts of geppetto that are not covered by the showcase nor the redux example application
It should be able to be used to automatically test if a change to the geppetto codebase (new feature / refactor, f.e) is a breaking change. This testing shouldn't be very detailed but rather focused on basic features (smoke test)

Where does this new application map with the other applications examples:

| Application | Meant to |
| ------------- | ------------- |
| Geppetto Showcase | Showcase geppetto UI features to the clients |
| React-Redux Example | Showcase how to use flex-layout and widgets to developers |
| Backend Example | Showcase how to use geppetto backend to developers |

How do we add this?
- List backend features to maintain and showcase
- Remove no longer maintained features of pygeppetto (like experiments, etc..)
- Add tornado + flask integration (done on mgv) to pygeppetto
- Document how to use maintained backend features
- Create simple frontend application (basic flex-layout; websocket api (load_project_by_url) + rest openapi)
- Add backend application to handle frontend requests (websocket + rest openapi)
- Create smoke tests for backend application

Start by inventorying the maintained pygeppetto backend features and comparing the existing Geppetto Showcase and React-Redux Example applications. The issue lists the intended frontend, websocket and REST OpenAPI integration, Flask/Tornado support, documentation, and smoke tests; done requires a maintained backend example covering these basic flows.

Written by the indexing model from the issue text.