Ensure that session is relocked if compositor or lock screen die

[jackpot51]

When #27 is merged, it will be possible to save a file to some path like /run/user/$UID/cosmic-greeter-$XDG_SESSION_ID to denote when the session is supposed to be locked. This file can be used to automatically relock the session if either the compositor or the lock screen died and were restarted.

[jokeyrhyme]

This might only be needed for the case that the compositor malfunctions

Provided the compositor is still functioning and implements https://wayland.app/protocols/ext-session-lock-v1 , the lock screen should be able to malfunction without the compositor considering the screen unlocked

[Drakulix]

The compositor doesn't unlock the screen, but cosmic-greeter needs to know on restart (after a potential malfunction), if it should run in the background or re-acquire the lock to display the lock screen.