[effects] Fix type of X-KWin-Video-Url

Summary:
The desktop file parser supports only QString, QStringList, Int, Double,
and Bool. QUrl is not a supported type for some reason.

Thus, let's use QString for X-KWin-Video-Url.

Reviewers: #kwin, broulik

Reviewed By: broulik

Subscribers: Zren, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15373