Use String to store UDS_USER and UDS_GROUP of String type.

Summary:
First crash I get after enabling Dr. Konqi for slaves.
UDS_USER and UDS_GROUP are strings, not numbers, therefore I've got an assert (simplified):
"udsField & KIO::UDSEntry::UDS_NUMBER", "udsentry.cpp", line=line@entry=113)

Use the same methods to get those data as in file.

BUG:398867

Test Plan: Search an obvious file in $HOME, it is shown.

Reviewers: dfaure, #baloo, #frameworks, ngraham

Reviewed By: dfaure, #baloo, ngraham

Subscribers: ngraham, broulik, kde-frameworks-devel

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D15635