[B+C] Add methods to get and set knockback strength in Arrow. Adds BUKKIT-5103
[B+C] Add setCritical and isCritical methods to Arrow.java, Adds BUKKIT-5113
[Bleeding] Fill in gaps of projectile related API. Addresses BUKKIT-1038, BUKKIT-391, BUKKIT-1536, BUKKIT-3060, BUKKIT-4214, BUKKIT-1156