LibCore: Introduce a C_OBJECT macro.

This macro goes at the top of every CObject-derived class like so:

class SomeClass : public CObject {
    C_OBJECT(SomeClass)
public:
    ...

At the moment, all it does is create an override for the class_name() getter
but in the future this will be used to automatically insert member functions
into these classes.

Libraries/LibCore/CoreIPCClient.h

@@ -49,6 +49,7 @@ private:
 
 template <typename ServerMessage, typename ClientMessage>
 class Connection : public CObject {
+    C_OBJECT(Connection)
 public:
     Connection(const StringView& address)
         : m_notifier(CNotifier(m_connection.fd(), CNotifier::Read))