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/Point.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <AK/AKString.h>
+
 class Rect;
 struct GUI_Point;
 
@@ -47,6 +49,7 @@ public:
     }
 
     operator GUI_Point() const;
+    String to_string() const { return String::format("[%d,%d]", x(), y()); }
 
 private:
     int m_x { 0 };

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;

SharedGraphics/Size.h

@@ -24,6 +24,11 @@ public:
                m_height == other.m_height;
     }
 
+    bool operator!=(const Size& other) const
+    {
+        return !(*this == other);
+    }
+
     operator GUI_Size() const;
 
 private: