[C+B] Add methods to get and set arrow knockback. Adds BUKKIT-5103
[B+C] Add setCritical and isCritical methods to CraftArrow.java, Adds BUKKIT-5113
Add files for diff visibility.
[Bleeding] Fill in gaps relating to projectiles API. Addresses BUKKIT-1038, BUKKIT-391, BUKKIT-1536, BUKKIT-3060, BUKKIT-4214, BUKKIT-1156