Take the screenlocker UI from the shell package

The screenlocker UI is very security-sensitive and can't be trusted to come from the store. Move it in the shell package instead, which is not supposed to be available in the store

In the long term we'll remove everything that is "code" from the lnf package, which will become a pure pool of settings

depends from: libplasma!1081 (merged) plasma-workspace!4112 (merged) plasma-desktop!2136 (merged) plasma-mobile!482 (merged)