Add 'time span' pseudo-columns

The calendar display has always seemed a little "off" to me, because of the way that entries are only displayed at points in time, and not the spans of time they represent in reality.
This mitigates that by allowing the search format to specify "time spans" like "Starts-Due" or "Started-Resolved". This way each ticket shows up for every day it is "active." - the way a traditional calendar app does.