Changeset 10787

#2106: Changed StableSort extension methods:

• Parameters index and count were ignored in List<T>.StableSort(int, int, Comparison<T>)
• Made both StableSortComparer<T> private classes
• Added sanity checks similar to those of Array.Sort and List<T>.Sort
  • Before it was possible that index + count was bigger than the size of the list
• Reduced number of method overloads to 2 by introducing IComparer<T> as optional parameter