Add Screen Lock options (moved to GNOME Privacy in Settings 3.8)

[Jeremy Bícha]

To make it easier to track, I'm re-focusing this bug on just a request to add the Screen Lock functions that gnome-control-center 3.8 has moved to their new Privacy panel.

These are the Screen Lock on/off switch and how long to wait after the screen goes blank before locking the screen.

I guess we can use bug 992338 to track the request to add the new features (Recently Used, time to retain recent items, Purge Trash & Temporary Files).

Original bug report
===================
GNOME 3.8 includes new Privacy and Search settings panels which offer similar but different functionality to Ubuntu's Privacy settings panel. There are several different ways Ubuntu's Privacy panel could go. I suggest incorporating pieces from GNOME's panel in the Privacy panel.

Specifically these settings:
- Screen Lock (otherwise 3.8 is a regression from 3.6 as these setttings have been moved to the new Privacy panel)
- Recently Used toggle switch and time to retain from Usage & History
- Purge Trash & Temporary Files

But not...
- Name & Visibility probably isn't too useful in Unity

This could be an extra set of tabs that would be added with a build-time configure switch... or you could build them and only show it if the (Ubuntu-specific sadly) XDG_CURRENT_DESKTOP environment variable =Unity.

[Manish Sinha (मनीष सिन्हा) (manishsinha)]

Better left for later work

[Manish Sinha (मनीष सिन्हा) (manishsinha)]

See #1198761 for last feature "Purge Trash & Temporary Files" for GNOME 3.8