Add caffeine quick setting

Using !1361 (merged) we can use shell functionality in plugins so let's use it to demo how simple custom quick settings will become once @arun-mani-j !1359 (merged) lands by implementing the caffeine quick setting.

When the quick setting is toggled it prevents the session from going idle.

All but the last commit are cleanups and documentation improvements.

Closes: #829 (closed)