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/GLabel.h

@@ -6,6 +6,7 @@
 class GraphicsBitmap;
 
 class GLabel : public GFrame {
+    C_OBJECT(GLabel)
 public:
     explicit GLabel(GWidget* parent = nullptr);
     GLabel(const StringView& text, GWidget* parent = nullptr);
@@ -26,8 +27,6 @@ public:
 
     void size_to_fit();
 
-    virtual const char* class_name() const override { return "GLabel"; }
-
 private:
     virtual void paint_event(GPaintEvent&) override;