The old API only allowed cancelling property "get" inquiries and
subscription updates. However, there are use-cases for cancelling other
requests too. e.g. switching between views in a JUCE app might mean that
it's no longer necessary to subscribe to a particular property.

Cancelling subscriptions ends up being quite involved. Different
handling is needed depending on whether the subscription is cancelled
before or after the responder replies to the initial request.
In addition, the responder may ask the initiator to retry a subscription
begin request.

The new class hold a list of ChildProcesses and periodically checks
their return value until they report termination. On Linux this check
is necessary to avoid leaving zombie processes behind.

This fixes an edge case in which if listeners are both removed and added during a callback the added listener(s) may be called during the same iteration

- Prevent out of scope access of the listeners lock
- Allow clearing the listener list from a callback

- Subsequently drops support for Xcode 15.0 while allowing users who have already resolved Xcode 15.0 issues to continue building

This also fixes an issue where MIDI CI header data could contain spaces,
which is not allowed according to the spec.

This change affects the DatagramSocket and StreamingSocket classes.

- Also refactored internals to use weak/shared pointers

This allows defining JUCE_LOG_CURRENT_ASSERTION when using Visual
Studio, where escaping semicolons in the command-line is not possible.