Adding centralized damage calculation function

[gmriggs]

The various CalculateDamage functions were getting a bit scattered, so this adds a common DamageEvent class / CalculateDamage() function that handles all scenarios

Previously, there were 3 separate variations of CalculateDamage:

• Players attacking monsters
• Players attacking other players (PvP)
• Monsters attacking players / monsters

This merges everything into 1 common function that handles everything for all physical damage calculation