Add a new value changed reason and comment the 3 main value change
reasons:

USER_EDITED = 0,     //< a user change triggered the call, gui will not
be refreshed but instanceChangedAction called
    PLUGIN_EDITED ,     //< a plugin change triggered the call, gui
will be refreshed but instanceChangedAction not called
    NATRON_EDITED, //<  Natron called setValue itself,
instanceChangedAction will be called AND gui refreshed