Unify property names for Inventory and InventoryItem

[Nordsoft91]

Currently item protoset property name is inconsistent: In Inventory class name is item_protoset In InventoryItem class name is protoset

Suggest to unify name

[peter-kish]

Yup, the naming is inconsistent. Sadly, it will break compatibility with previous versions if fixed, so it will have to be implemented in the next major release (in fact it's already implemented on the dev_v3.0.0 branch).

[peter-kish]

Since version 3.0.0 the property names are aligned