NativeMessaging manifest not installed on macOS

[jelmervdl]

See jelmervdl/translatelocally-web-ext#53. I can reproduce this using the build linked there. No manifest files are written on start-up, nor when I manually call translateLocally with the --update-manifests option.

[jelmervdl]

It seems the macOS version is run from a sandbox, so manifests go into the translatelocally container. Not sure why this happens on other macs, but not my own.

I need to figure out a way to ask the user for consent to escape that container. There are APIs for this (.e.g full disk access, or asking access to the user's home folder should be sufficient really) but I'm not sure how to access those from Qt yet.

One thing that then should definitely change is that we need to not install manifests on launch, but after the user clearly indicated intent. Related to #107 and #108 (which has a mock-up)

[jelmervdl]

Another thing to try (combined with the button): Can I use getExistingDirectory to trigger the macOS Sandbox to ask permission to the ~/Library/Application Support folder? And then write all the manifests I want to write?