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/CSocket.h

@@ -6,6 +6,7 @@
 class CNotifier;
 
 class CSocket : public CIODevice {
+    C_OBJECT(CSocket)
 public:
     enum class Type {
         Invalid,
@@ -35,8 +36,6 @@ public:
 
     Function<void()> on_connected;
 
-    virtual const char* class_name() const override { return "CSocket"; }
-
 protected:
     CSocket(Type, CObject* parent);