Package for macOS

[micahflee]

Right now dangerzone will run in macOS from the source tree, but I need to add actual Mac packaging to create an app bundle and an installer pkg. The app bundle should register itself to be able to open pdf, docx, etc. file formats.

Also while I'm at it, it would be cool if I can turn on the macOS sandbox. The tricky part is needing to be able to subprocess to docker and open, and also being able to read and write to the correct places in the filesystem. But if I can make it work, that would be great. Dangerzone itself doesn't need network access, too -- only docker does to pull the container.

[micahflee]

The app bundle is capable of opening all of the supported file formats in master now. And I confirmed that, so long as we're relying on Docker Desktop, the sandbox isn't an option (#8).

But actually, I think it might be nice to release this as a dmg instead of a pkg, just to slightly reduce the friction to installing it, because dmg installers don't need root.