add switchUser to ksmserver's dbus API

this will allow us to move switch user functionality from krunner to ksmserver
as ksmserver manages the sessions (ergo its name) and the lock screen, this is
a logical place to put it. it also means we end up with just one UI (in the lock
screen greater) that is fully themable (in the QML sense: interaction, looks, etc)
for user sesssion and guarantee that the current sesssion is locked before we
switch to another session