Provide a mechanism for binding actions on session lock/unlock events

[roosemberth]

Originally motivated by #7007

User automation can benefit from knowing whether a session is locked. For example, automation can stop a time-tracking software when the screen is locked and prompt the user whether they'd like to continue the tracker when the session is unlocked.

systemd-logind provides support for this, but it needs to be notified about this hint.

@kennylevinsen proposed a solution which could be reused by platforms not running (e)loggind:

If we need lock hooks, we should just have a form of event bind - one my event go as far as calling the command bindevent - so you can trigger something when lock/unlock happens.

The comment is a bit ambiguous, but the way I understand it is to have a new command bindevent (similar to bindsym or bindswitch), and users could run an arbitrary command (eg. dbus-send --system --dest=org.freedesktop.login1 /org/freedesktop/login1/session/self org.freedesktop.login1.Session.SetLockedHint boolean:false to address the original issue).

[Vladimir-csp]

either that or extend bindswitch to support other events:

• session_lock:on|off|toggle
• output:connected|disconnected|changed
• docked:on|off|toggle
• input:connected|disconnected|changed

[emersion]

I'd prefer to expose new IPC events, instead of adding bindevent. As a matter of fact IPC events already exist for outputs and inputs.