That feature is a poor man's session management for applications
that do not implement any specific session management features.
It badly interferes with proper session management support, so
applications must be able to disable it.

This enables fixing applications with
QGuiApplication::quitOnLastWindowClosed() true - the default -
dying too early, before they are enumerated for the list of
applications to restart on session restore, thus preventing them
from being restored. See
https://bugs.kde.org/show_bug.cgi?id=354724



[ChangeLog][QtGui] Qt asking to close windows on session exit as
a fallback session management mechanism has been made optional.
Disabling it fixes session management for applications that
implement full session management. See
QGuiApplication::isFallbackSessionManagementEnabled().

Task-number: QTBUG-49667
Change-Id: Ib22e58c9c64351dea8b7e2a74db91d26dd7ab7aa
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>