Add instant lock

jurgenhaas → credited drubb → .

jurgenhaas → credited rkoller → .

I've implemented this to provide the "Lock screen" in the toolbar and in the user account menu. It also binds Ctrl-Alt-L to immediately lock the screen.

@drubb and @rkoller maybe you can give this a try? It's in the latest dev-release.

What's missing:

• An icon is required for the toolbar
• The toolbar and the navigation module are not using the user account menu to show the user related links. I remember that @rkoller knew from where they're coming instead, but I can't find it again. Do you remember?