Lock screen support (`light-locker`)

[tuurep]

Using https://jezerm.github.io/web-greeter-page/docs/api/Greeter.html#lightdm-lock-hint

• Change "Login" to "Unlock"
• What about choosing a different user/session in lock screen?
  • Either:
    • Disable selecting another user/session
      • This depends on localStorage not working properly in web-greeter #2
    • Show "Unlock" for active user&session, "Login" for other user/session
      • How to know if a session is active (for a given user&session combo)?

[tuurep]

I'm gravitating towards disabling the selections.

However, problems:

• Caching last user and session must work
• What if there are for example 2 different user sessions running concurrently?
  • Could it show the wrong one in the lock screen if screen locked from the session that was logged in first?

[tuurep]

Getting lock_hint has some issues in web-greeter, see discussion in: JezerM/web-greeter#76

Personally I have replaced light-locker with xsecurelock, and feel like it's close to ideal.