Sets or unsets the "action" and "target" attributes of this.
If action is null then both the "action" and "target" attributes are unset (and
             format_string is ignored along with the positional parameters).
If action is non-null then the "action" attribute is set. format_string
             is then inspected. If it is non-null then the proper position parameters are 
            collected to create a Variant instance to use as the target value. If it is 
            null then the positional parameters are ignored and the "target" attribute is unset.
See also set_action_and_target_value for an equivalent call that directly accepts a Variant. See set_detailed_action for a more convenient version that works with string-typed targets.
See also set_action_and_target_value for a description of the semantics of the action and target attributes.
| this | a MenuItem | 
| action | the name of the action for this item | 
| format_string | a GVariant format string | 
| ... | positional parameters, as per  |