Fixed callback not fired on slidein toggle

Was trying to understand fx options and looked at the slideToggle-function - there i saw the original callback being stored in a variable which was never used (well, netbeans saw it for me and was so kind to notify me :grinning: ).

Trying out the function i saw my test callback only being executed every second time - bam! Works now on every toggle as one would expect, hope this change makes sense repectively follows the authors intention.