Add GitHub Actions support

- add GitHub Action to build and test cxx plugin: cxx-ci.yml
- add automatic release step: 'Latest Snapshot'
- add GitHub Action badge to README.md
- ignore HTML Tidy warnings
- optimize behave step 'the analysis log contains a line matching': support all line ending combinations [\r\n]
- adapt 'stop_sonar' to work with GitHub Action
- add 'requirements.txt' for behave in the integration folder
- remove DeepCode badge
- use SQ 8.9.7.52159
- delete Travis and AppVeyor support