Item frames with maps

This is part of #790 but I decided to split it because implementing map rendering doesn't require us to render items. I guess it would be an entity with a dynamic texture, similar to how signs are implemented in Chunky.

Quoting @NotStirred from Discord:

> iirc maps are stored in the world/data directory,
> would need to read the map entity data to get the map number, then find the saveddata and get the map colour array from there
> will check how it works in older versions one moment
> right... previously (pre flattening) the map item's damage was used to determine which map it was
very intuitive