Move the message definitions to their own files.

xfa/fwl/core/ifwl_datetimeedit.h

 // 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
 
 #ifndef XFA_FWL_CORE_IFWL_DATETIMEEDIT_H_
 #define XFA_FWL_CORE_IFWL_DATETIMEEDIT_H_
 
-#include "xfa/fwl/core/cfwl_message.h"
 #include "xfa/fwl/core/cfwl_widgetproperties.h"
 #include "xfa/fwl/core/fwl_error.h"
 #include "xfa/fwl/core/ifwl_edit.h"
 #include "xfa/fwl/core/ifwl_widget.h"
 
 class IFWL_DateTimeEdit : public IFWL_Edit {
  public:
   IFWL_DateTimeEdit(const IFWL_App* app,
                     std::unique_ptr<CFWL_WidgetProperties> properties,
                     IFWL_Widget* pOuter);
 
   // IFWL_Edit.
   void OnProcessMessage(CFWL_Message* pMessage) override;
 
  private:
   void DisForm_OnProcessMessage(CFWL_Message* pMessage);
 };
 
 #endif  // XFA_FWL_CORE_IFWL_DATETIMEEDIT_H_