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/GTreeView.h

@@ -3,12 +3,12 @@
 #include <LibGUI/GAbstractView.h>
 
 class GTreeView : public GAbstractView {
+    C_OBJECT(GTreeView)
 public:
     explicit GTreeView(GWidget*);
     virtual ~GTreeView() override;
 
     virtual void scroll_into_view(const GModelIndex&, Orientation);
-    virtual const char* class_name() const override { return "GTreeView"; }
 
 protected:
     virtual void paint_event(GPaintEvent&) override;