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.

Applications/PaintBrush/ColorDialog.h

@@ -3,10 +3,10 @@
 #include <LibGUI/GDialog.h>
 
 class ColorDialog final : public GDialog {
+    C_OBJECT(ColorDialog)
 public:
     explicit ColorDialog(Color, CObject* parent = nullptr);
     virtual ~ColorDialog() override;
-    virtual const char* class_name() const override { return "ColorDialog"; }
 
     Color color() const { return m_color; }
 

Applications/PaintBrush/PaintableWidget.h

@@ -4,14 +4,13 @@
 class Tool;
 
 class PaintableWidget final : public GWidget {
+    C_OBJECT(PaintableWidget)
 public:
     static PaintableWidget& the();
 
     explicit PaintableWidget(GWidget* parent);
     virtual ~PaintableWidget() override;
 
-    virtual const char* class_name() const override { return "PaintableWidget"; }
-
     Color primary_color() const { return m_primary_color; }
     Color secondary_color() const { return m_secondary_color; }
 

Applications/PaintBrush/PaletteWidget.h

@@ -5,12 +5,11 @@
 class PaintableWidget;
 
 class PaletteWidget final : public GFrame {
+    C_OBJECT(PaletteWidget)
 public:
     explicit PaletteWidget(PaintableWidget&, GWidget* parent);
     virtual ~PaletteWidget() override;
 
-    virtual const char* class_name() const override { return "PaletteWidget"; }
-
     void set_primary_color(Color);
     void set_secondary_color(Color);
 

Applications/PaintBrush/ToolboxWidget.h

@@ -3,11 +3,8 @@
 #include <LibGUI/GFrame.h>
 
 class ToolboxWidget final : public GFrame {
+    C_OBJECT(ToolboxWidget)
 public:
     explicit ToolboxWidget(GWidget* parent);
     virtual ~ToolboxWidget() override;
-
-    virtual const char* class_name() const override { return "ToolboxWidget"; }
-
-private:
 };