Feature: block notifications unless Mac has been idle for specified time

[mkosma]

Add a parameter using MacOS HIDIdleTime to allow user to specify that no notifications should be delivered unless user input has been idle for some amount of time (perhaps 30 seconds by default) to prevent breaking concentration with popups.

For an example, see https://www.dssw.co.uk/blog/2015-01-21-inactivity-and-idle-time/