[KDirOperator] Highlight the previous dir when going back/up
We have a couple of cases:
- starting in a/, going to b/, then back(), b is highlighted
- starting in a/b/, cdUp to "a", "b" is highlighted
- starting in a/b/c/, cdUp twice to "a", back(), "c" is highlighted
This is useful for orienting oneself while navigating in kfilewidget, especially in big directories.
Add unit test to kdiroperatortest.
BUG: 422748 FIXED-IN: 5.72