- 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.

- Refactored code to remove repetition and magic numbers

Now that VST3 builds are signed before and after manifest generation, it
may be necessary to force-sign in order to overwrite the
previously-applied signature.

Co-authored-by: Tatsuya Shiozawa <shio.tatsu99@gmail.com>

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.

Co-authored-by: Roland Rabien <figbug@gmail.com>