LibGUI: Start working on a GScrollBar.

This widget is far from finished, but it's off to a good start.
Also added a GResizeEvent and GWidget::resize_event() so that widgets
can react to being resized.

SharedGraphics/Rect.h

@@ -2,6 +2,7 @@
 
 #include "Point.h"
 #include "Size.h"
+#include <AK/AKString.h>
 
 struct GUI_Rect;
 
@@ -171,6 +172,8 @@ public:
 
     Rect united(const Rect&) const;
 
+    String to_string() const { return String::format("[%d,%d %dx%d]", x(), y(), width(), height()); }
+
 private:
     Point m_location;
     Size m_size;