histedit: add forceother constraint
For the future 'base' action in histedit we need a verification
constraint which will not allow using this action with changes
that are currently edited.
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.