histedit: make verification configurable
Before we can add a 'base' action to histedit need to change verification so that action can specify which steps of verification should run for it. Also it's everything we need for the exec and stop actions implementation. I thought about baking verification into each histedit action (so each of them is responsible for verifying its constraints) but it felt wrong because: - every action would need to know its context (eg. the list of all other actions) - a lot of duplicated work will be added - each action will iterate through all others - the steps of the verification would need to be extracted and named anyway in order to be reused The verifyrules function grows too big now. I plan to refator it in one of the next series.