Add EVENT_TICK for NPCs

This event fires at the start of the tick processing so we can
script on the actual tick.

zone/event_codes.h

@@ -82,6 +82,7 @@ typedef enum {
 	EVENT_RESPAWN,
 	EVENT_DEATH_COMPLETE,
 	EVENT_UNHANDLED_OPCODE,
+	EVENT_TICK,
 
 	_LargestEventID
 } QuestEventID;