Adds holder to the list of holders managed within set.
NOTE: if set already has a #GdaHolder with the same ID as holder, then holder will not be added to the set (even if holder's type or value is not the same as the one already in set).
a #GdaHolder object
TRUE if holder has been added to set (and FALSE if it has not been added because there is another #GdaHolder with the same ID)
Adds holder to the list of holders managed within set.
NOTE: if set already has a #GdaHolder with the same ID as holder, then holder will not be added to the set (even if holder's type or value is not the same as the one already in set).