Start refactoring graphics system to have per-window backing stores.

It was fun for everyone to share a single framebuffer but it was also
kinda really awful. Let's move towards having a "GraphicsBitmap" as the
backing store for each Window.

This is going to need a lot of refactoring so let's get started.

Widgets/Widget.cpp

@@ -1,6 +1,7 @@
 #include "Widget.h"
 #include "Event.h"
 #include "EventLoop.h"
+#include "GraphicsBitmap.h"
 #include "WindowManager.h"
 #include "Window.h"
 #include "Painter.h"
@@ -155,3 +156,10 @@ void Widget::setFont(RetainPtr<Font>&& font)
     else
         m_font = std::move(font);
 }
+
+GraphicsBitmap* Widget::backing()
+{
+    if (auto* w = window())
+        return w->backing();
+    return nullptr;
+}