* Rename creature template `heroic_entry` => `difficulty_entry_1`
Possible need add 2 more fields for 2-3
* Use 2 more EventAI EFLAG_* flags for 2-3 difficulty support, update checks.
* Update area trigger code.
Note: support for specialized creature version for difficulties 2-3 not added yet
and will be used same as difficulty 1 mode.
TODO: Rename area triggers heroickey/etc fields ?