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/AbstractButton.h

@@ -26,14 +26,10 @@
 
 #pragma once
 
-#include <LibCore/Timer.h>
-#include <LibGfx/TextAlignment.h>
 #include <LibGUI/Widget.h>
 
 namespace GUI {
 
-class Painter;
-
 class AbstractButton : public Widget {
     C_OBJECT_ABSTRACT(GAbstractButton)
 public: