[libkwineffects] Use correct type for the desktops property

Summary:
Type of the x11DesktopIds property was changed from QList<int> to
QVector<int> in KWin core, but not in libkwineffects. That resulted in
having effects like slide not working because Qt can't implicitly
convert QVector<int> to QList<int>.

Test Plan: Slide effect works again.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16743