LibWeb+LibJS: Move DOM Window object to dedicated classes

LibWeb now creates a WindowObject which inherits from GlobalObject.
Allocation of the global object is moved out of the Interpreter ctor
to allow for specialized construction.

The existing Window interfaces are moved to WindowObject with their
implementation code in the new Window class.

Libraries/LibWeb/Forward.h

@@ -35,15 +35,21 @@ class Event;
 class EventListener;
 class EventTarget;
 class Frame;
+class HTMLBodyElement;
 class HTMLCanvasElement;
 class HTMLElement;
+class HTMLHeadElement;
+class HTMLHtmlElement;
 class HtmlView;
+class LayoutDocument;
 class LayoutNode;
 class MouseEvent;
 class Node;
 class Selector;
+class StyleResolver;
 class StyleRule;
 class StyleSheet;
+class Window;
 
 namespace Bindings {
 
@@ -56,6 +62,7 @@ class EventTargetWrapper;
 class HTMLCanvasElementWrapper;
 class MouseEventWrapper;
 class NodeWrapper;
+class WindowObject;
 class Wrappable;
 class Wrapper;