feat: New template structure

[fussel178]

Summary

First setup for the new and discussed Telestion application structure.

Details

Updated things:

• merge Java Apps and PSCs into one repository (see chore: Merge PSC with this repo telestion-rocketsound#1)
• add test workflow for core app
• improved Docker publish workflow
  • now uses the new GitHub Container Registry to push Docker images users can use without access to the repo
  • automatic generation from commit messages via Conventional Commits
• switch to Java 16
• add some default/nice-to-have configuration files
• updated READMEs

Intended Release Cycle

When you want to release a new application, go to the Actions Tab in the GitHub UI and choose the Release Action.
Then click Run workflow.

This triggers the Release Action which automatically creates a Conventional Commit Release on GitHub.
Afterwards the subsequent build actions are triggered via:

on:
  workflow_run:
    workflows: ["Release"]
    types: [completed]

Additionally, if you want to upload release assets in these build workflows, you can use the cached build environment:

      - name: Restore cache release upload URL ♻️
        uses: actions/cache@v2.1.5
        with:
          path: ~/.build-env
          key: 'github-release-action'

      - name: Import environment
        run: cat ~/.build-env >> $GITHUB_ENV

      - name: Upload release asset
        uses: actions/upload-release-asset@v1.0.1
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        with:
          upload_url: ${{ env.upload_url }}
          asset_path: ./dist/artifact1.txt
          asset_name: artifact1.txt
          asset_content_type: text/plain

You can find more information and an example here:
https://github.com/fussel178/conventional-release-dispatch

Feel free to comment and make improvements on this PR.
I'm open for suggestions. 😉

[jvpichowski]

LGTM

[jvpichowski]

core -> application
gui -> client

[fussel178]

@jvpichowski and @pklaschka Thank you very much for your contributions! 🍹