Summary:
Instead of having a separate switch user screen, we just invoke the lock
screen with a parameter to default to the switch session page.

This is better because:

- it's a familiar consistent UI to the lockscreen

- It has all the lockscreen features. (Wallpapers, Multiscreen, OSDs)

- Session switching is simpler as we won't have to wait to invoke the
lock screen before switching VTs

- We get rid of one more chunk of rubbish code from ksmserver
(it becomes just KSldApp::self()->lock(DefaultToSwitchUser) instead of a
blocking dialog)

- There's one less lnf component to maintain

However, it does bring the behavioural change that you can't cancel from
the "switch user screen" without entering a password

(argubaly a feature... Switch User locks on some other platforms)

And the session will unconditionally lock when you switch session,
currently it's linked to the config option "Lock screen automatically"

(which is arguably completely wrong anyway, as that config option is
displayed as being about locking after a timeout)

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2525