Stopped Filling Empty Dispensers

Changed FillContainersOperation so that empty dispensers are not filled
with a stack of arrows on import. Just in case someone needs to use
empty dispensers in their design. As far as I can remember, this won't
affect any of our dungeons - arrow traps are rare. The only dungeon that
I can remember is one by Balgor and it was exported with all of its
dispensers loaded.
Updated How to Add Dungeons

Updated our dungeon creation guide. It was badly outdated by now.