Make wxDumpWindow() public and available in wxMSW too

This function was defined in wxGTK and wxOSX, but not in wxMSW or the
other ports, but it can be useful there too, so make it public and
define it in common code.

include/wx/window.h

@@ -2100,6 +2100,9 @@ extern WXDLLIMPEXP_CORE wxWindow *wxGetActiveWindow();
 // get the (first) top level parent window
 WXDLLIMPEXP_CORE wxWindow* wxGetTopLevelParent(wxWindowBase *win);
 
+// Return a string with platform-dependent description of the window.
+extern WXDLLIMPEXP_CORE wxString wxDumpWindow(wxWindowBase* win);
+
 #if wxUSE_ACCESSIBILITY
 // ----------------------------------------------------------------------------
 // accessible object for windows