[feature request] Add zip archiving functionality to Markor

[gsantner]

A frequent point of question is how to backup files of Markor. Many users use some sync software like Syncthing or alike which has server-side backup functionalities as well.

What however would be a great addition to Markor is to be able to manually zip up some directory (i.e. notebook) so the outcoming is one zip file that can be used e.g. for exchange with others or to backup stuff.

For this, Java has builtin java.util.Zip classes, so no additional dependency is needed for this functionality.