Move the message definitions to their own files.

xfa/fwl/core/ifwl_comboedit.cpp

 // Copyright 2016 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_comboedit.h"
 
 #include "xfa/fde/cfde_txtedtengine.h"
+#include "xfa/fwl/core/cfwl_msgmouse.h"
 #include "xfa/fwl/core/ifwl_combobox.h"
 
 IFWL_ComboEdit::IFWL_ComboEdit(
     const IFWL_App* app,
     std::unique_ptr<CFWL_WidgetProperties> properties,
     IFWL_Widget* pOuter)
     : IFWL_Edit(app, std::move(properties), pOuter) {
   m_pOuter = static_cast<IFWL_ComboBox*>(pOuter);
 }
 
@@ skipping 42 matching lines @@
         m_pOuter->SetFocus(true);
       }
       break;
     }
     default:
       break;
   }
   if (backDefault)
     IFWL_Edit::OnProcessMessage(pMessage);
 }