Refactor inventory items out of images.json

[evktalo]

Inventory items that are added to inventory over the course of adventure are currently defined in images.json, and are added in their own invisible Konva Layer to the stage.

Can we get rid of this extra Layer?

Could we add them to the inventory layer "on the fly" as they are picked up / created to the inventory?

[evktalo]

Instead of having a layer of their own, could the inventory items (when invisible) reside in the inventory layer?

Is it necessary for them to reside in a layer at all? Can they exist in the stage in some other way?

Could they be created on the fly, instead of at the start?

[evktalo]

Minimal implementation in 770bae0