[C] Make Slimes fire EntityTarget events. Fixes BUKKIT-1408

### The Issue:
Currently, Slimes do not fire the appropriate EntityTargetEvent when targeting players.

### Justification for this PR:
As CraftBukkit has an event that is called in other places where entities are targeted by entities, it seems appropriate that they also fire this event here.

### PR Breakdown:
This PR adds the appropriate call to a new EntityTargetEvent event when a slime targets a new player. It will not fire a new event if the slime targets the same player it targeted on the previous tick.

### JIRA Ticket:
BUKKIT-3915 - https://bukkit.atlassian.net/browse/BUKKIT-1408