Fix detection of window ID

Summary:
Session cookies weren't correctly deleted on shutdown because the window
ID passed to KCookieServer::addCookies was always 0. Now, the correct id
is passed.

Test Plan:
go to a page which creates session cookies (for example GMail), then
exit and open the Cookies KFM. Look at the cookies and check that there aren't
session cookies.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: broulik

Differential Revision: https://phabricator.kde.org/D23353