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/LibGUI/GInputBox.h

@@ -6,14 +6,13 @@ class GButton;
 class GTextEditor;
 
 class GInputBox : public GDialog {
+    C_OBJECT(GInputBox)
 public:
     explicit GInputBox(const StringView& prompt, const StringView& title, CObject* parent = nullptr);
     virtual ~GInputBox() override;
 
     String text_value() const { return m_text_value; }
 
-    virtual const char* class_name() const override { return "GInputBox"; }
-
 private:
     void build();
     String m_prompt;