Queries the type of the parameter that must be given when activating
the named action within @action_group.
When activating the action using g_action_group_activate_action(),
the #GVariant given to that function must be of the type returned
by this function.
In the case that this function returns %NULL, you must not give any
#GVariant, but %NULL instead.
The parameter type of a particular action will never change but it is
possible for an action to be removed and for a new action to be added
with the same name but a different parameter type.
Queries the type of the parameter that must be given when activating the named action within @action_group.
When activating the action using g_action_group_activate_action(), the #GVariant given to that function must be of the type returned by this function.
In the case that this function returns %NULL, you must not give any #GVariant, but %NULL instead.
The parameter type of a particular action will never change but it is possible for an action to be removed and for a new action to be added with the same name but a different parameter type.