[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