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/GTextBox.h

@@ -4,9 +4,8 @@
 #include <LibGUI/GTextEditor.h>
 
 class GTextBox final : public GTextEditor {
+    C_OBJECT(GTextBox)
 public:
     explicit GTextBox(GWidget* parent);
     virtual ~GTextBox() override;
-
-    virtual const char* class_name() const override { return "GTextBox"; }
 };