Instanced entities should have unique ids

[bitbrain]

While we have to preserve the original id internally, it makes sense that instances get their own entity ids. Reason being is that for inventory systems, two items may share the same instanced entity id (e.g. "Gold Hammer") but both hammers may be completely different due to gameplay progression, so we need a way to distinguish between them. Either:

• entity id always reflects Pandora entity id and a new attribute called instance_id is introduced (which is null by default)
• entity id is unique for instanced entities, although this might be confusing to know where it originated from, thus we need to expose the "instanced from" entity id