Thorium release workflow

In a nutshell:

• everything works on the develop branch (tested in Windows, Linux and MacOS)
• create new CHANGELOG markdown (which will form the basis for the notes in the GitHub tagged release)
• version bump in package.json, src/package.json and package-lock.json
• create and push release tag (see the "Git release tag for GitHub releases" page in this wiki)
• merge into master (git checkout master && git merge develop && git push)
• come back to develop branch, version bump to new "alpha" (e.g. from v1.7.0 to v1.7.1-alpha.1), git commit -a -m "chore(release): version bump alpha" && git push

Plaftorms:

• On Linux, npm run package:linux generates a Debian package and an AppImage which are both to be uploaded to the GitHub release. There is no code signing.
• On MacOS, npm run package:mac looks for a signing certificate in KeyChain, and there is notarization via Apple's servers (long upload time). The generated DMG is to be uploaded to the GitHub release.
• On Windows, npm run package:win generates an unsigned APX (which will be signed during App Store submission), and an unsigned NSI installer (EXE).

See the "Code Signing" page in this wiki for further information.