More Progress, Inventory, 1.16.

Horses are flying better in the new update to the mod for 1.15.2 - I've been reworking the logic of that code to clean it up and make it easier to understand (plus the movement was choppy in 1.15.2). Right now I'm working on getting the horse inventory working, since the mod needs that to work to access a lot of the things you can do with magical horses. I plan to release a version when the main things are working - horses moving around and interacting with the world correctly, breeding working, main horse inventory screen working. Custom horses may or may not make it into my first release After that, depending on what people want, I'll polish that release a bit or move on to getting a version out for 1.16.