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/AbstractView.h

@@ -27,14 +27,11 @@
 #pragma once
 
 #include <AK/Function.h>
-#include <LibGUI/Model.h>
 #include <LibGUI/ModelSelection.h>
 #include <LibGUI/ScrollableWidget.h>
 
 namespace GUI {
 
-class ModelEditingDelegate;
-
 class AbstractView : public ScrollableWidget {
     C_OBJECT_ABSTRACT(AbstractView)
     friend class Model;