AI-Event throwing and receiving implementation

Add new AI hooks to
* SendAIEvent (either around the npc, or to a specified npc)
* ReceiveAIEvent (hook to receive sent events)
Add ACTION_T_THROW_AI_EVENT and EVENT_T_RECEIVE_AI_EVENT to EventAI

A few heads and tails i see:
* An event usually has: Sender + Invoker, so we might need a new target-type to support both
* most eventAI Events are not triggered with an TARGET_T_INVOKER, so it is unclear how we could ie:
  Throw Event CRITICAL_HEALTH on 5% health - so for a healer the event-sender is good to know (and already known)
  But for a dd maybe the event-invoker (or dmg-dealer) would be better choice for knowing and to place an action onto him

* Do we need more AIEvent types for EventAI (maybe even switch to uint32, where first byte == sd2, second byte == eventAi, third byte = db-scripts forth-byte=core)
* Which parts currently in the core can be transferred to this system?
  As example I have in mind to replace SD2's dynamic casts for like starting an escort with event sending (EVENT_START_ESCORT)