[libkwineffects] Use correct type for the desktops property
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
Differential Revision: https://phabricator.kde.org/D16743