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/AbstractTableView.cpp

@@ -25,9 +25,11 @@
  */
 
 #include <AK/StringBuilder.h>
+#include <AK/Vector.h>
 #include <LibGUI/AbstractTableView.h>
 #include <LibGUI/Action.h>
 #include <LibGUI/Menu.h>
+#include <LibGUI/Model.h>
 #include <LibGUI/Painter.h>
 #include <LibGUI/ScrollBar.h>
 #include <LibGUI/Window.h>