LibGUI: Add forwarding header

This patch adds <LibGUI/Forward.h> and uses it a bunch.
It also dragged various header dependency reduction changes into it.

Libraries/LibGUI/Layout.h

@@ -26,16 +26,14 @@
 
 #pragma once
 
-#include <AK/Badge.h>
 #include <AK/OwnPtr.h>
 #include <AK/Vector.h>
 #include <AK/WeakPtr.h>
+#include <LibGUI/Forward.h>
 #include <LibGUI/Margins.h>
 
 namespace GUI {
 
-class Widget;
-
 class Layout {
 public:
     Layout();