Mod items in inventory

[Pepe20129]

This PR allows for runtime registration of custom items with callbacks to retrieve the item icon, item name texture and item action func.

Item name texture currently doesn't work and more advanced items (items that have more complex behaviour that just execute code on use like bow, hammer, etc.) can't be implemented yet in this system.

Build Artifacts

[garrettjoecox]

Going to close this, the documentation changes, and some of the changes to the save editor are welcome to come in a separate PR, but I think most of the code here falls under the more invasive mod territory than we want in the main codebase for now. I think plenty of people will still be interested in this code so I would recommend keeping the branch around and creating a post in the modding forum if you haven't already.