Like Sequence.sort, but uses a GSequenceIterCompareFunc instead
of a GCompareDataFunc as the compare function
cmp_func is called with two iterators pointing into seq. It should
return 0 if the iterators are equal, a negative value if the first
iterator comes before the second, and a positive value if the second
iterator comes before the first.
the function used to compare iterators in the sequence
user data passed to cmp_func