Manually delete PowerDevil Core on teardown

The QObject children cleanup routine is run after QGuiApplication's destructor after which
the X connection has already been cleaned up. By manually deleting in PowerDevilApp's
destructor, we delete Core first before any Qt cleanup happens.

CHANGELOG: PowerDevil no longer crashes on logout

BUG: 371127
FIXED-IN: 5.8.3

Differential Revision: https://phabricator.kde.org/D3152