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/GButton.h

@@ -11,6 +11,7 @@ class GAction;
 
 class GButton : public GAbstractButton {
 public:
+    GButton(const String& text, GWidget* parent);
     explicit GButton(GWidget* parent);
     virtual ~GButton() override;