prepared code for #24

- Refactored Selection & SimpleSelection to become Collection
independent
- Therefore added Iterable interface
- Renamed collections clone method to copy because of method conflict
(Object has private clone method)
- Wrote JUnit test for SimpleSelection which proved that SimpleSelection
1. works and 2. preserves element order.
- Implemented new enum TaskSortAttribute which defines a task Comparator
for every attribute used for sorting tasks.
- Implemented SortForm and prototyped sort GUI.