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/CHttpJob.h

@@ -7,14 +7,13 @@
 class CTCPSocket;
 
 class CHttpJob final : public CNetworkJob {
+    C_OBJECT(CHttpJob)
 public:
     explicit CHttpJob(const CHttpRequest&);
     virtual ~CHttpJob() override;
 
     virtual void start() override;
 
-    virtual const char* class_name() const override { return "CHttpJob"; }
-
 private:
     void on_socket_connected();