Make wxPG_FILE_DIALOG_STYLE a built-in attribute

This attribute is used only internally in wxFileProperty so it 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/props.h

@@ -589,6 +589,7 @@ protected:
     wxString    m_initialPath; // If set, start the file dialog here
     wxString    m_dlgTitle; // If set, used as title for file dialog
     int         m_indFilter; // index to the selected filter
+    long        m_dlgStyle;  // File dialog style
 };
 
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