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/GCheckBox.h

@@ -5,6 +5,7 @@
 #include <LibGUI/GAbstractButton.h>
 
 class GCheckBox : public GAbstractButton {
+    C_OBJECT(GCheckBox)
 public:
     GCheckBox(const StringView&, GWidget* parent);
     explicit GCheckBox(GWidget* parent);
@@ -12,8 +13,6 @@ public:
 
     virtual void click() override;
 
-    virtual const char* class_name() const override { return "GCheckBox"; }
-
 private:
     // These don't make sense for a check box, so hide them.
     using GAbstractButton::auto_repeat_interval;