Add original wxPropertyGridXmlHandler code

This code was included in wxPropertyGrid standalone library before it
had been included in wxWidgets itself.

It doesn't compile yet, but will soon.

include/wx/xrc/xh_propgrid.h

@@ -0,0 +1,55 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        xh_propgrid.h
+// Purpose:     XML resource handler for wxPropertyGrid
+// Author:      Jaakko Salli
+// Modified by:
+// Created:     May-16-2007
+// RCS-ID:      $Id:
+// Copyright:   (c) Jaakko Salli
+// Licence:     wxWindows license
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_XH_PROPGRID_H_
+#define _WX_XH_PROPGRID_H_
+
+/*
+
+  NOTE: relevant source file, xh_propgrid.cpp is *not* included in the
+    wxPropertyGrid library (to prevent xrc-lib dependency). To use this
+    code, you will need to separately add src/xh_propgrid.cpp to your
+    application.
+
+*/
+
+#include "wx/xrc/xmlres.h"
+
+#include "wx/propgrid/propgrid.h"
+#include "wx/propgrid/manager.h"
+
+#if wxUSE_XRC
+
+class wxPropertyGridXmlHandler : public wxXmlResourceHandler
+{
+    friend class wxPropertyGridXrcPopulator;
+    DECLARE_DYNAMIC_CLASS(wxPropertyGridXmlHandler)
+
+public:
+    wxPropertyGridXmlHandler();
+    wxObject *DoCreateResource() override;
+    bool CanHandle(wxXmlNode *node) override;
+
+    void InitPopulator();
+    void PopulatePage( wxPropertyGridState* state );
+    void DonePopulator();
+
+    void HandlePropertyGridParams();
+
+private:
+    wxPropertyGridManager*      m_manager;
+    wxPropertyGrid*             m_pg;
+    wxPropertyGridPopulator*    m_populator;
+};
+
+#endif // wxUSE_XRC
+
+#endif // _WX_XH_PROPGRID_H_