CI and release building

All branches described at the Wiki page "Git commit workflow" are automatically build checked via continuous integration (CI):

• Pull requests aiming at the devel branch are build checked by a GitHub-actions build-runner when posing a PR and for each new commit to the source branch as long as a PR is open.
• ToDo: The master branch is build checked as harbour-storeman-testing at the SailfishOS-OBS for each pull request to it (from the devel branch, which is its only source branch).
• Pull requests aiming at the release branches {sfos3.2,sfos3.3,sfos4.2} (primarily from the masterbranch) are build checked by a GitHub-actions build-runner when posing a PR and for each new commit to the source branch as long as a PR is open.
• ToDo: The release branches sfos3.2, sfos3.3 and sfos4.2 branch is build checked as harbour-storeman-testing at the SailfishOS-OBS for each pull request to them at the harbour-stroreman-ci subproject
• The release branches sfos3.2, sfos3.3 and sfos4.2 are build as harbour-storeman-sfosX.Y packages (each for a non-overlapping range of SFOS release targets) by a GitHub-actions build-runner at GitHub after a new tag has been set. They are ultimately build as releases at the SailfishOS-OBS when this tag has been copied to their _sources files at OBS.

An overview of the build repositories and their detailed structure is depicted in chapter 1.1 here: https://github.com/storeman-developers/harbour-storeman/issues/266