No changes, just extract wxCreateHiddenWindow() declaration in a header.

Add wx/msw/private/hiddenwin.h with wxCreateHiddenWindow() declaration instead
of declaring it manually in all the files using it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69170 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/private/hiddenwin.h

@@ -0,0 +1,31 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/msw/private/hiddenwin.h
+// Purpose:     Helper for creating a hidden window used by wxMSW internally.
+// Author:      Vadim Zeitlin
+// Created:     2011-09-16
+// RCS-ID:      $Id: wxhead.h,v 1.12 2010-04-22 12:44:51 zeitlin Exp $
+// Copyright:   (c) 2011 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_MSW_PRIVATE_HIDDENWIN_H_
+#define _WX_MSW_PRIVATE_HIDDENWIN_H_
+
+#include "wx/msw/private.h"
+
+/*
+  Creates a hidden window with supplied window proc registering the class for
+  it if necessary (i.e. the first time only). Caller is responsible for
+  destroying the window and unregistering the class (note that this must be
+  done because wxWidgets may be used as a DLL and so may be loaded/unloaded
+  multiple times into/from the same process so we can't rely on automatic
+  Windows class unregistration).
+
+  pclassname is a pointer to a caller stored classname, which must initially be
+  NULL. classname is the desired wndclass classname. If function successfully
+  registers the class, pclassname will be set to classname.
+ */
+extern "C" WXDLLIMPEXP_BASE HWND
+wxCreateHiddenWindow(LPCTSTR *pclassname, LPCTSTR classname, WNDPROC wndproc);
+
+#endif // _WX_MSW_PRIVATE_HIDDENWIN_H_