Enable Download Client for MAC

Apple currently prohibits opening jar files provided by the download client.

To open the jar-files in a MAC environment, the Java application would have to be notarized by Apple. The notarization should not be a big deal and is described in this article:

https://www.joelotter.com/2020/08/14/macos-java-notarization.html

Therewith MAC users would also be able to havethe full functionality of ARIS Connect.