Bug#658883: [gnome-shell] Reproduce on fresh Debian Wheezy Install

[Georg Wicherski]

Installed Debian wheezy _without_ the task-desktop meta-package and the
interesting packate (gdm3, gnome-shell, gnome-screensaver) manually.
Using the option in the system menu works, but using Ctrl+Alt+L does not.

When opening the shortcuts dialog, the shortcut appears to be already
defined to Ctrl+Alt+L. Redefining it (by clicking the column and
pressing Ctrl+Alt+L) makes it work now.