Accumulate restart requests to show the notification at transaction end
Depending on the backend onRestartRequired might get called during the transaction or even when it begins. Prompting for restart/logout while a transaction is running should be avoided. This also makes sure that the notification is not shown for simulated transactions.
Fixes https://bugzilla.opensuse.org/show_bug.cgi?id=1103679 and http://bugzilla.opensuse.org/show_bug.cgi?id=1174239