Move the message definitions to their own files.

xfa/fwl/core/ifwl_listbox.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "xfa/fwl/core/ifwl_listbox.h"
 
 #include "third_party/base/ptr_util.h"
 #include "xfa/fde/tto/fde_textout.h"
-#include "xfa/fwl/core/cfwl_message.h"
+#include "xfa/fwl/core/cfwl_msgkey.h"
+#include "xfa/fwl/core/cfwl_msgmouse.h"
+#include "xfa/fwl/core/cfwl_msgmousewheel.h"
 #include "xfa/fwl/core/cfwl_themebackground.h"
 #include "xfa/fwl/core/cfwl_themepart.h"
 #include "xfa/fwl/core/cfwl_themetext.h"
 #include "xfa/fwl/core/ifwl_app.h"
 #include "xfa/fwl/core/ifwl_themeprovider.h"
 
 namespace {
 
 const int kItemTextMargin = 2;
 
@@ skipping 1081 matching lines @@
     Repaint(&m_rtClient);
   }
   return true;
 }
 
 void IFWL_ListBox::DispatchSelChangedEv() {
   CFWL_EvtLtbSelChanged ev;
   ev.m_pSrcTarget = this;
   DispatchEvent(&ev);
 }