The Paper method was chosen for deprecation because it was more restrictive in that it has an isGliding check.
* fix PotionAPI ignores icon flag * also fix CraftPotionUtil#toBukkit * also CraftPotionUtil#fromBukkit * use CraftPotionUtil
* Fix PlayerSwapHandItemsEvent throwing exception when mainhand or offhand set to null * use fully qualified import * Use ItemStack#empty() instead of new ItemStack * Add NotNull annotation to getters * Add missing Paper comments