[StatJob] Use A QFlag to specify the details returned by StatJob

Summary: Use this QFlag to minimize fields retrieved by statx, yielding a little memory and performance yield.

Test Plan: ctest

Reviewers: Frameworks, dfaure, kossebau

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25010