Doing window closures here means that clients die before the session and window managers, e.g., KDE ksmserver and kwin_x11, are unable to include them in their saved states, breaking session management (see for example, https://bugs.kde.org/show_bug.cgi?id=346768).

It would seem more appropriate to not worry about session cancellation here, allowing clients to do this is their commitData() slots.

The attached patch, plus some minor patching in KDE5's kxmlgui and plasma-workspace, at least in my testing, allows KDE5 session management to work as expected.