Make wxPG_ATTR_MULTICHOICE_USERSTRINGMODE a built-in attribute

This attribute is used only internally in wxArrayStringProperty and thus can be considered as a built-in attribute which value can be stored in the local data member and not in the property's attribute store.

include/wx/propgrid/advprops.h

@@ -344,6 +344,7 @@ public:
                                int argFlags = 0) const wxOVERRIDE;
     virtual bool OnEvent( wxPropertyGrid* propgrid,
                           wxWindow* primary, wxEvent& event ) wxOVERRIDE;
+    virtual bool DoSetAttribute( const wxString& name, wxVariant& value ) wxOVERRIDE;
 
     wxArrayInt GetValueAsArrayInt() const
     {
@@ -361,6 +362,8 @@ protected:
 
     // Cache displayed text since generating it is relatively complicated.
     wxString            m_display;
+    // How to handle user strings
+    int                 m_userStringMode;
 };
 
 #endif // wxUSE_CHOICEDLG