Demos: Start working on a simple WidgetGallery app.

It's good to have a place where we can try out all the different widgets.
This needs some more work on a nice layout, and should also include more
of the widgets. :^)

LibGUI/GCheckBox.h

@@ -6,6 +6,7 @@
 
 class GCheckBox : public GAbstractButton {
 public:
+    GCheckBox(const String&, GWidget* parent);
     explicit GCheckBox(GWidget* parent);
     virtual ~GCheckBox() override;