RFC: Move Session Switching to the Display Manager

[David Edmundson]

One of the goals of the Look and Feel package is to make all session
related things consistent, given we're talking about depending on
functionality in the DM can I propose we move session switching there
too.

Practically every Display Manager  already has some session switching
built in, I would like to :
 - drop the current session/user switching (the list that appears in krunner)
 - change it so when one clicks the the "Switch user" button in
kickoff it just switches to the display manager.
 - same for the lock screen

It's just as many clicks for the user, and it saves having duplicate
code and two very different UIs to do exactly the same thing.

If the lock screen is watching logind session status correctly it
should deactivate when the session is activated, so we won't have two
login prompts or anything.

David