[kcoredirlister] Port away from QLinkedList

Summary: Use QVector instead. Not sure that's the best thing complexity-wise

Test Plan: kcoredirlister benchmark passes

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12087

Differential Revision: https://phabricator.kde.org/D25446