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/GToolBar.cpp

@@ -45,6 +45,7 @@ void GToolBar::add_action(GAction& action)
 }
 
 class SeparatorWidget final : public GWidget {
+    C_OBJECT(SeparatorWidget)
 public:
     SeparatorWidget(GWidget* parent)
         : GWidget(parent)
@@ -63,9 +64,6 @@ public:
         painter.draw_line({ 0, 0 }, { 0, rect().bottom() }, Color::MidGray);
         painter.draw_line({ 1, 0 }, { 1, rect().bottom() }, Color::White);
     }
-
-private:
-    virtual const char* class_name() const override { return "SeparatorWidget"; }
 };
 
 void GToolBar::add_separator()