ROOMBA_TASKS: Bugfixes

- Fixed adding and removing walls when zoomed. Items are asked
  with map_->scene()->items() instead of just map_->items().
  This maps the verticelocations given as input correctly to the
  scene they reside in instead of the current viewed part of
  mapQGraphicsView.
- Fixed calculating new POI in poiCollected. Previously
  bestDistance was never updated from the max value so the last POI
  was always chosen as nearest.