Implement Farthest Hostile target in Scripts editor and Spell List ed…

[balakethelock]

Additions to Script Editor that make it compatible with https://github.com/vmangos/core/pull/2352

Adds "target farthest hostile" target type to spell list and AI script. Searches for the farthest enemy in the creature's threat list (If the script is to cast a spell, the search range is capped at the range of the spell)