Draft of lyrics-related content types updated according to review feedback

Reorder declarations to work properly with C++20 std::unique_ptr<> requirements

Move semaphore from MainThreadMessageDispatcher to dedicated helper class and refactor dispatch accordingly

waitForMessage...() must be implemented by IPC MEssageChannel instead of Connection to properly deal with IPC running on the main thread

Suppress unnecessary main thread wakeups when waiting for IPC messages

AudioUnit implementation of AUMessageChannel moved from example code to IPC library as dynamic ObjC class

Made IPC MessageDispatcher and its subclasses private details of Connection implementation

IPC ReplyHandler encapsulated into std::function<>

Instead of subclassing, provide IPC Connection delegate data as construction parameter

IPC Connection encapsulates WaitForMessageDelegate as std::function<>

IPC code uses std::unique_ptr<> to express ownership

Fix double delete of IPC Channel in AUv3 case

Make MessageHandler a creation parameter of IPC Connection

Simplify IPC debug logging

IPCProxyPlugIn/Host take ownership of its associated IPCConnection