FileSystem: Fix using creation time from KIO as changed time (ctime)

Added new field to FileItem instead. And do not show dummy value in UI for
missing file times - also for other time types.

FIXED: [ 397398 ] Misleading ‘Created’ date in file properties
BUG: 397398

Differential Revision: https://phabricator.kde.org/D20741

(cherry picked from commit 0920b04b)