Move session management code from QApplication to QGuiApplication.

Move session management code from QApplication to QGuiApplication.

QSessionManager was already in QtGui, but not usable since the only API
to get to it was still in QtWidgets. Session management isn't related
to widgets, it also applies to QML apps on the desktop.

The virtual commitData and saveState methods have been removed,
given the two signals which exist since 4.2, and an additional
isSessionSaving() method was added instead.

Change-Id: I8099e70df133303e74456706827da21f013fcc6a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>