KFileItem: remove separate storage of times, use UDSEntry instead.

Test Plan:
Added unittest to check that the UDS fields are filled in the exact
same way by kio_file and KFileItem::init (especially for symlinks)

Reviewers: Frameworks, aacid

Reviewed By: aacid

Subscribers: aacid

Differential Revision: https://phabricator.kde.org/D1465