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/GSpinBox.h

@@ -6,6 +6,7 @@ class GButton;
 class GTextEditor;
 
 class GSpinBox : public GWidget {
+    C_OBJECT(GSpinBox)
 public:
     GSpinBox(GWidget* parent = nullptr);
     virtual ~GSpinBox() override;
@@ -21,8 +22,6 @@ public:
 
     Function<void(int value)> on_change;
 
-    virtual const char* class_name() const override { return "GSpinBox"; }
-
 protected:
     virtual void resize_event(GResizeEvent&) override;