[Feature Request] Add an option to the SDDM greeter to choose either displaying avatars or asking for username

[talmage]

As an individual concerned with privacy, I want to hide the valid usernames on my computer. The current implementation displays avatars remembers the username of the last user to log in.

In order to hide the valid usernames,; the greeter must not display them and must require each user to type in both username and password.

There seems to be code for this in Login.qml but there is no obvious way to enable it.