Scripts: Implemented virtual function DoCastSpellOnPlayers (self-explanatory) and CheckRequiredBosses checking states of other bosses before allowing combat with selected one

Scripts/Icecrown Citadel: Added checks to prevent skipping bosses

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -545,7 +545,15 @@ void BossAI::_EnterCombat()
     me->setActive(true);
     DoZoneInCombat();
     if (instance)
+    {
+        // bosses do not respawn, check only on enter combat
+        if (!instance->CheckRequiredBosses(bossId))
+        {
+            EnterEvadeMode();
+            return;
+        }
         instance->SetBossState(bossId, IN_PROGRESS);
+    }
 }
 
 void BossAI::TeleportCheaters()