[Feature] Support condition in listener configuration

[Diaoul]

The goal is to run a condition (or test_cmd) to know if the listener should trigger or not.

Use case: Based on whether my laptop is plugged or on battery I want different behaviors and avoid putting that logic in the on-timeout and on-resume as tests.

Current workaround:
I prefix my on-resume and on-timeout commands:

[ "$(cat /sys/class/power_supply/AC/online)" = "0" ] && brightnessctl ...

However, it's not perfect: I don't want the on-resume to run if in between I've plugged/unplugged my laptop. In other words, if the on-timeout did not trigger because of condition, I don't want the on-resume to run at all.