[KListOpenFilesJob] Use QString::splitRef()

Summary:
As per apol's suggestion in D26210, use splitRef(). And Since it returns
a QVector<String>, use std::unique to remove the duplicates.

Test Plan: make && ctest

Reviewers: Frameworks, mpyne, dfaure, apol

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26226