Add e2e tests with testcafe

[Bogdan-Lyashenko]

https://devexpress.github.io/testcafe/documentation/getting-started/

1. add test:e2e script to package.json which runs testcafe for tests/e2e dir (create dir as well); tests should be run against codedrumbs running at PLAYGROUND_URL (set env var as localhost in webpack.dev config)
2. add page-objects dir in tests/e2e/, put files with selectors there (ID-s for UI controls as well as for SVG tags for main elements should be added)
3. cover basic user flows with tests - change UI configurations and verify if correct diagram is being rendered