make equipping rideable animals more intuitive

Rideable animals are not always clearly communicating to the player they can get accessories. Players who don't know they can equip a mule by clicking a chest on it will never know about this feature for example. I would propose adding an extra equip slot in the UI of a rideable pet to show these options. Just like how horses have a slot for horse armor, mules should get a slot for chests. The same should go for llamas and any other mob that has a not-yet-clearly-communicated ability to equip an item.