Allowed rooms to be built over litter/vomit

Dealing with issue #147 this commit's change is based on what
happens in TH when rooms are built over litter/vomit.

This commit makes tiles with litter/vomit (vomit is a litter type)
on them buildable while a blueprint is over them and then deletes
litter which is in the blueprint when it is confirmed.

New functions added because this commit requires them:
 World.lua:
 +findObjects()
 +findObjectsInArea()
 Utilty.lua:
 +set_insert()
 +set_remove()
 +set_get_missing_from()