added code signing and notariziation

[timschneider]

Added codesigning again and notarization of dmg file.

Set the following variables and secrets in git

MACOS_CERTIFICATE: ${{ secrets.MACOS_P12 }} <- the certificate base64 encoded
MACOS_CERTIFICATE_PWD: ${{ secrets.MACOS_PASSWD }} <- the certificate passphrase
MACOS_APP_PWD: ${{ secrets.MACOS_NOTARIZATION_PASS }} <- the apple developer id (application specific) password
MACOS_APP_MAIL: ${{ secrets.MACOS_NOTARIZATION_USER }} <- the apple developer id (email)
MACOS_APP_DEVELOPER_ID: ${{ vars.MACOS_APP_DEVELOPER_ID }} <- the apple developer id subject name (as written in certificate)
MACOS_APP_TEAM_ID: ${{ vars.MACOS_APP_TEAM_ID }} <- the apple id team id used for notarization
MACOS_KEYCHAIN_TEMP_PASSWORD: ${{ secrets.MACOS_KEYCHAIN_TEMP_PASSWORD }} <- a strong random password

[gretel]

@supermerill did you try building for macos via github actions? it should be possible, i think.