Lock session after time, rather than suspending

### Problem

* I know this isn't strictly a Greeter issue, but greeter was the closest relevant component. Please re-file this under the appropriate repo.

In Jolnir, when the PC is left alone for a period, the system is suspended after 30 minutes. Suspend is pretty heavy-handed. Ideally, a video conference should continue to run and continue sending audio to a wireless headset, while it won't be possible to directly interact with the machine until having logged in a second time. This should also work when a session is locked manually.

In addition, it's a problem that when entering a session after suspension, the session is not actually restored, it's a fresh login.

### Proposal

There needs to be a concept of a locked session, which would return to the greeter, window locations should be serialized and saved to a file somewhere, and the processes spawned by the session should be kept alive. Upon restore, the serialized window locations should be processed and used to put things back where they were.

I believe lightdm supports some version of this upstream, from a quick search of that codebase.

### Prior Art (Optional)

Can't speak to the specific implementation details, but if I'm logged into a Windows session, it does behave this way. It will lock the session after a period of inactivity, but continue to run the processes associated with that session. OSX will also do this, and it seems likely Ubuntu is happy to do this too.