introduce a 'locked' property for the qml lock screen

if password locking is turned off, this lets the locker not show a
password unlocker (e.g.)

if password locking is on but there is a grace period, then the
property gets set once the grace period times out and the locker
can show the unlock UI

Conflicts:
ksmserver/screenlocker/greeter/greeterapp.cpp
ksmserver/screenlocker/greeter/themes/org.kde.passworddialog/contents/ui/main.qml