[xembedsniproxy] Free windowAttributes in every case

The structure was only freed in case the XTest method was chosen.
But it should always be freed of course (as long as it isn't NULL).

Use a QScopedPointer instead to free it automatically when it goes out
of scope and to be more consistent with the rest of t

Differential Revision: https://phabricator.kde.org/D6389