FIX #8188 Properly show link for showing and hiding class spec in model admin

On a side note, I had to use slideToggle as opposed to the usual toggle method, which is not available in that scope. I don't understand why but it seems related to a jquery sub definition of the $ variable (using jQuery.('#bla).toggle works, but not $('#bla).toggle();) If someone has a clue, I'd like to understand why, but that's optional, as slideToggle is way more cool B-)