[Bleeding] Fix logic for calculating slot in Creative mode. Fixes BUKKIT-4715

Previously, hotbar slots for player inventory would return 9 - 18 while in
Creative mode, rather than 0 - 9. This commit fixes the logic used for
calculating the returned slot based on the raw slot.
Add getDamage and to Boat class. Adds BUKKIT-5325

The Boat entity is missing the getDamage method found on
the Minecart entity. Without it, it is impossible to determine the
health of a boat. This commit adds the aforementioned method, allowing
the health of a Boat to be retrieved.