Call QCoreApplication::exit instead of stdlib exit to terminate if platform fails

Summary:
We need to properly tear down the application - this can be achieved
through QCoreApplication::exit. Otherwise there is a chance that the
cleanup handling crashes.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2136