swayidle with higher precision

[Artoria2e5]

the date command now prints to nanosecond, though obviously we won't expect anything that good.

Also, fix an off-by-one-sec error. The printed time is always one second after the last-active time because of how we configured sway.

[Artoria2e5]

I don't want to flood you with PRs, so here's a list of things I want to change but don't think is worth a PR:

• which is not an universally available program. using shell command -v xprintidle is more portable.
• get_monitor is not used by get_dbus_idle. this probably should not be the case.