Use inventory instrinsic width if not specified in inventory-window opts

Requires deathcap/inventory@4c7ea9df3c361a35b229d07ae2fdf257e05fefa7
Rename test to demo
Update demo for 2D inventory change

https://github.com/deathcap/inventory/commit/4c7ea9df3c361a35b229d07ae2fdf257e05fefa7
Update README to link to demo