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.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <AK/StringBuilder.h>
+#include <AK/Vector.h>
 #include <Kernel/KeyCode.h>
 #include <LibGUI/AbstractView.h>
 #include <LibGUI/DragOperation.h>