Introduces a new arena setting that changes the "emptiness check" for
`clear-wave-before-next`, `clear-wave-before-boss`, and the final wave
check, allowing for a customizable _leeway_.
By default, the leeway is 0, which means the monster set has to be
completely empty for the checks to pass. With a value of 2, the set
may contain up to two monsters for the checks to pass, and so on.
The leeway should help alleviate some of the issues some people have
with their arena sessions when monsters "disappear" behind terrain or
end up in hard-to-reach areas. This is by no means a real "solution"
to the underlying problem, since the build-up of monsters in longer
sessions will just result in the issue being pushed to later waves.
We'll see if the setting leaves any additional customization to be
desired and perhaps defer any such requests to the Sessions Rework.
Closes#706
