* Core/Unit: Allow to define Units which can enter water but cannot swim
Allow to define Units which can enter water but cannot swim, i.e. crabs walking at the bottom of a sea.
* Add UNIT_FLAG_SWIMMING to creatures when entering combat
* Fix charmed creatures not entering water
* Always allow Creatures controlled by players to enter water
* Add swimming flag when possessing a unit and remove it properly at the end, even if the creature engaged combat before and after.
When adding/removing UNIT_FLAG_SWIMMING manually calling Creature::RefreshSwimmingFlag(true) might be required.