LibCore: Add a class for thread-safe promises

Since the existing Promise class is designed with deferred tasks on the
main thread only, we need a new class that will ensure we can handle
promises that are resolved/rejected off the main thread.

This new class ensures that the callbacks are only called on the same
thread that the promise is fulfilled from. If the callbacks are not set
before the thread tries to fulfill the promise, it will spin until they
are so that they will run on that thread.

Userland/Libraries/LibCore/Forward.h

@@ -36,6 +36,8 @@ class ProcessStatisticsReader;
 class Socket;
 template<typename Result, typename TError = AK::Error>
 class Promise;
+template<typename Result, typename TError = AK::Error>
+class ThreadedPromise;
 class SocketAddress;
 class TCPServer;
 class TCPSocket;