filenamesearch: Implement stat to display metainfo

Summary:
filenamesearch ioslave needed a stat implementation to be rendered properly in dolphin information panel.

BUG: 420354
FIXED-IN: 20.08

Test Plan: Open filenamsearch:/ in dolphin and search something

Reviewers: Dolphin, Frameworks, ngraham, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D29197