Add Release Pipeline

[ingomohr]

This is based on a discussion w/ @AObuchow .

For now, we have a CI that builds, tests and deploys the updatesite for every branch to a dedicated build-branch.

Those branches can help for development, but they don't cover all we need to cover for a release.

So, here are some ideas the release pipeline could cover:

• Integrate w/ the "release" option on Github
  • i.e. We could use the "release" option to create a new release out of the e.g. the master branch.
  • The "release" event would trigger the release pipeline
• The pipeline takes the fully-tested master-branch-build (available atm. on branch "updatesite - which is currently used as release updatesite location)
• ... and adds it to a composite updatesite that contains all release-versions of the theme
  • The composite updatesite would need a fitting location on some server
• The pipeline also could add the changelog to the Github release entry - see Add GitHub Workflow for generating changelog #89
• If possible, the pipeline could also update the Eclipse Marketplace entry for the theme

[AObuchow]

I'm +1 for all these ideas.

I have no experience with the release option of GitHub but it'd be cool to use it if possible :)

We could probably use https://bintray.com/ to host the composite updatesite.

Integrating this with #89 would be awesome :D Very fitting.

As for updating the Eclipse Marketplace entry, I'm not sure yet if there's an API to do that. @PyvesB do you know if this is possible? I quickly checked https://wiki.eclipse.org/Marketplace/REST#General_Usage but it seems to be directed towards consuming marketplace listings, rather than modifying them.

[PyvesB]

I don't think it's possible with Marketplace API v1, though worth confirming with someone from the Foundation. However, I believe this will be possible with the Marketplace API v2, see the docs here.