jQuery toggle script
You can either write a method that is bound to the global scope and takes two arguments, the onClick element and the slideToggle element, and does just what you want (without writing redundant code).

function bindToggleTwo(cElem, tElem) {
   $(cElem).click(function() {
      $(tElem).slideToggle(1200, 'easeInOutQuart');
      $(".hideable").slideToggle(1200, 'easeInOutQuart');
   });
}

jQuery(document).ready(function($) {
    bindToggleTwo("#alles", ".abutton");
    bindToggleTwo("#voorbeelden", ".bbutton");
    bindToggleTwo("#contact", ".cbutton");
});


Alternatively, you could write a jQuery function that is called on a jQuery object to do the very same. Basically this is a function that expects to be scoped such that this is  a jQuery object. Anyway, both of these methods work, and will simplify your code. :)

jQuery.fn.bindToggleTwo = function(tElem) {
  $(this[0]).click(function() {
     $(tElem).slideToggle(1200, 'easeInOutQuart');
     $(".hideable").slideToggle(1200, 'easeInOutQuart');
  });
}

jQuery(document).ready(function($) {
    $("#alles").bindToggleTwo(".abutton");
    $("#voorbeelden").bindToggleTwo(".bbutton");
    $("#contact").bindToggleTwo(".cbutton");
});