[wayland] Move AbstractClient::desktops from QList to QVector

Summary:
Doesn't have any meaningful impact. It's the same performance when T is a pointer,
but it'll bring it consistent with VirtualDesktopManager::desktops

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16736