Don't delete unused object properties

This lets us use objects as associative arrays.
Added framework for migrating to later versions

This will fix issues for people upgrading from very old
versions, and allow more radical changes to internal data
structures.
Fix overview when migrating from version 4.x
Migrate to the new item system fully, and do it only once
Removed deprecated items and added monster food to the list of displayed items
Merge pull request #9 from m101010ax/pushes

Pushes