- Also refactored internals to use weak/shared pointers

When dragging-and-dropping a plugin into the plugin host, it was
possible that the subprocess scanner could fail to send a response.
This would cause the superprocess to freeze, waiting for a reply.

With this change in place, the subprocess will always send a response
after scanning on the main thread.

To prevent too much disruption to people who have our public projects as
part of their build or testing systems we are temporarily ignoring a
warning about using Xcode 15.

This allows real-time threads to join an audio workgroup on Apple platforms.

- added unit tests
- best performance timers used for each platform
- fixed an issue in which timer callbacks could drift

This reverts commit 82377a787a.