Idea #3859: Warn on shutdown when multiple users logged in

Multiple users can be logged in by using the user switching. But when the second user forgets about the first one and tries to shut down the computer, no warning pops up and it kills all the sessions silently. That's really bad, because all the unsaved work of the first user is completely lost. It would be *so easy* just to pop up confirmation dialog saying, that other users are still logged in and the shutdown can lose their unsaved data. In the fast-user-switch-applet we can see users currently logged in, so this is really a matter of few lines of code to implement. It would often save a lot of cursing. Windows have this functionality.

Example: Alice is working with Ubuntu. Bob wants to read his email quickly. Alice fast-switches to Bob account. Bob reads his email, forgets about Alice's running session and shuts down the computer. All Alice's unsaved work is lost. This wouldn't happen with the confirmation dialog.