AudioServer: Begin work on a new IPC API style.

The goal here is to generate most of this code from IPC protocol
descriptions, but for now I've spelled them all out to get started.

Each message gets a wrapper class in the ASAPI_Client or ASAPI_Server
namespace. They are convertible to and from the old message structs.

The real hotness happens when you want to make a synchronous request
to the other side:

    auto response = send_sync<ASAPI_Client::GetMainMixVolume>();

Each request class knows his corresponding response class, so in the
above example, "response" will be an ASAPI_Server::DidGetMainMixVolume
object, and we can get the volume like so:

    int volume = response.volume();

For posting messages that don't expect a response, you can still use
post_message() since the message classes are convertible:

    post_message(ASAPI_Server::DidGetMainMixVolume(volume));

It's not perfect yet, but I already really like it. :^)

Libraries/LibCore/CoreIPCClient.h

@@ -167,6 +167,18 @@ namespace Client {
             return response;
         }
 
+        template<typename RequestType, typename... Args>
+        typename RequestType::ResponseType send_sync(Args&&... args)
+        {
+            bool success = post_message_to_server(RequestType(forward<Args>(args)...));
+            ASSERT(success);
+
+            ServerMessage response;
+            success = wait_for_specific_event(RequestType::ResponseType::message_type(), response);
+            ASSERT(success);
+            return response;
+        }
+
     protected:
         struct IncomingMessageBundle {
             ServerMessage message;