macOS AUv3 needs import, export and recording manager like the iOS version #111
Description
When we implemented import, export and recording manager in the AUv3, it was only needed for iPadOS, because AUv3 support for macOS wasn't really tested and advertised much. This is about to change, so we must make the AUv3 for macOS just as proper as the iPadOS one. This is includes working around the fact that AUv3 (contrary to AUv2) runs in a sandbox and presents the same challenges with regard to exchanging files with the outside world, including between aumu, aumf and standalone.