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/BoxLayout.cpp

@@ -25,6 +25,8 @@
  */
 
 #include <LibGUI/BoxLayout.h>
+#include <LibGUI/Widget.h>
+#include <LibGfx/Orientation.h>
 #include <stdio.h>
 
 //#define GBOXLAYOUT_DEBUG