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/GResizeCorner.h

@@ -1,12 +1,11 @@
 #include <LibGUI/GWidget.h>
 
 class GResizeCorner : public GWidget {
+    C_OBJECT(GResizeCorner)
 public:
     explicit GResizeCorner(GWidget* parent);
     virtual ~GResizeCorner() override;
 
-    virtual const char* class_name() const override { return "GResizeCorner"; }
-
 protected:
     virtual void paint_event(GPaintEvent&) override;
     virtual void mousedown_event(GMouseEvent&) override;