Add support for Horse inventories (#2558)
* Can now open horse inventory
Command still needs testing and needs to be updated to use `autoExecute`
* Update command to use `autoExecute`
* Move errors and fix slot default value
* Change order of types
`public static` sounds better than `static public` in my opinion
* Fix `expire:` argument throwing an error when it shouldn't
* Cast to `CraftAbstractHorse` and throw exception to prevent NPE
* Update error messages
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commit: 74f28ba)
