Adds a method to get the arrow stack in EntityShootBowEvent and added
methods to cancel stack consumption. Adds BUKKIT-4686

Getting the ItemStack of an arrow that is being shot from a bow is
currently not possible, this PR allows this to be possible by adding a
method to EntityShootBotEvent that returns the stack in question.

This PR also adds methods to cancel the consumption of the stack. These
methods can override vanilla behaviours that would normally cancel the
consumption too. For example, infinity enchantments or creative mode.