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/WindowServerConnection.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <AK/SharedBuffer.h>
+#include <LibCore/EventLoop.h>
 #include <LibCore/MimeData.h>
 #include <LibGUI/Action.h>
 #include <LibGUI/Application.h>