Sets the target value of an actionable widget.
NULL then the target value is unset.
The target value has two purposes. First, it is used as the parameter
to activation of the action associated with the
Second, it is used to determine if the widget should be rendered as
“active” — the widget is active if the state is equal to the given target.
Consider the example of associating a set of buttons with a
with string state in a typical “radio button” situation. Each button
will be associated with the same action, but with a different target
value for that action. Clicking on a particular button will activate
the action with the target of that button, which will typically cause
the action’s state to change to that value. Since the action’s state
is now equal to the target value of the button, the button will now
be rendered as active (and the other buttons, with different targets,