Migrate CI to GitHub Actions

[LinqLover]

Closes #101 and closes #97.

This PR:

• migrates the entire CI pipeline from Travis CI to GitHub Actions, motivated by quota considerations, platform coolness & problems with Slack notifications as described in #97:

  

• decouples single CI jobs for testing, @TEST assertions in TEX, and screenshot/PDF generation for a faster and more fine-granular feedback on GitHub (see above)

  • this involves a split-up of the .smalltalk.ston files and related scripts
  • also extracts internal SBE tests into a separate package called SBE-Tests

• implements support for GitHub Actions CI in Makefile and SmalltalkSources

• adds Slack notifications if a scheduled build (currently run once a week) fails:

  

• provides PDF builds as artifacts on GitHub Actions instead of uploading them to Google Docs or GitHub Releases any longer:

  

• fixes the robustize-figures mode (aka DEBUG_FIGURES) which was broken due to an update of the graphicx package and since 85b2080fb30b40c0292174dba1bfc4e924e8878a (thanks again to @AxelKrypton for helping with this issue!)

• updates the documentation in the README accordingly.

Please squash when merging.

[LinqLover]

Artifacts such as PDF are only accessible when all jobs have been completed, reported here: https://github.com/actions/upload-artifact/issues/181

[LinqLover]

Slack notifications are failing, opened an issue here: https://github.com/voxmedia/github-action-slack-notify-build/issues/46

[LinqLover]

Message from smalltalkCI:

Unable to parse Coveralls build data from /home/runner/.smalltalkCI/_builds/coveralls_build_data.json:
STONReaderError: At character 465: 'invalid escape character \i'
This is a smalltalkCI internal error. If the problem persists, please open an issue at https://github.com/hpi-swa/smalltalkCI/issues/new .

Todo: If this problem persists, let's create an issue.

[LinqLover]

Todos before merging:

• [x] remove all debugging artifacts
• [x] merge #103 and make sure to get a green bar afterward
• [x] check both PDFs for hick-ups and version mismatches
• [x] squash when merging
• [x] Update docs
• [x] after merging: adjust required checks in repository settings

[codeZeilen]

Wow, nice work @LinqLover ! Do you need a detailed review of specific parts or just a general, sweeping look? :)

[LinqLover]

Wow, nice work @LinqLover ! Do you need a detailed review of specific parts or just a general, sweeping look? :)

Sweeping look should be enough :)