heads-up-display: Add extension for showing persistent heads up display message

Some locked down systems (like machines used for Top Secret documents, etc) want to have a way to show a message on screen most of the time when the user is using the system. Even when logging in and unlocking the screen.

This is what I came up to that end, based on a mockup from Allan.

Note, in order for it to work at the lock screen and login screen, it requires gnome-shell!1967 (merged) to land first.