Use KFileMetaData for XAttr support instead of private reimplementation

Summary:
This removes a lot of duplicate code, and on top it actually works
as it no longer hardcodes a number of locations (e.g. /tmp/ was always
blacklisted).

Test Plan: ctest

Reviewers: Baloo, ngraham, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D29181