Core/Spells: spell effect handling improvements

* Call spell effect handlers in 4 modes:
   - SPELL_EFFECT_HANDLE_LAUNCH - called when spell is launched (cast just finished)
   - SPELL_EFFECT_HANDLE_LAUNCH_TARGET - called when spell is launched for each target in spell target map
   - SPELL_EFFECT_HANDLE_HIT - called when spell hits its destination
   - SPELL_EFFECT_HANDLE_HIT_TARGET - called when spell hits it's target from spell target map
*Correctly implement SPELL_EFFECT_TRIGGER_SPELL, SPELL_EFFECT_TRIGGER_SPELL_WITH_VALUE, SPELL_EFFECT_TRIGGER_MISSILE_SPELL_WITH_VALUE, SPELL_EFFECT_TRIGGER_MISSILE_SPELL
*Remove spell system hacks which became obsolete with this commit
Core/SpellScripts:
add OnEffectLaunch, OnEffectLaunchTarget, OnEffectHit, OnEffectHitTarget hooks for new effect handle modes and remove OnEffect hook.
A generic rule of thumb how to update your scripts (will work for nearly all cases) for spell system noobs:
if your spell script used GetHitXXXX function, you need to use OnEffectHitTarget, otherwise use
OnEffectHit

src/server/scripts/Spells/spell_dk.cpp

@@ -233,7 +233,7 @@ class spell_dk_corpse_explosion : public SpellScriptLoader
 
             void Register()
             {
-                OnEffect += SpellEffectFn(spell_dk_corpse_explosion_SpellScript::HandleDummy, EFFECT_0, SPELL_EFFECT_DUMMY);
+                OnEffectHitTarget += SpellEffectFn(spell_dk_corpse_explosion_SpellScript::HandleDummy, EFFECT_0, SPELL_EFFECT_DUMMY);
             }
         };
 
@@ -266,7 +266,7 @@ class spell_dk_ghoul_explode : public SpellScriptLoader
 
             void Register()
             {
-                OnEffect += SpellEffectFn(spell_dk_ghoul_explode_SpellScript::Suicide, EFFECT_1, SPELL_EFFECT_SCHOOL_DAMAGE);
+                OnEffectHitTarget += SpellEffectFn(spell_dk_ghoul_explode_SpellScript::Suicide, EFFECT_1, SPELL_EFFECT_SCHOOL_DAMAGE);
             }
         };
 
@@ -307,7 +307,7 @@ class spell_dk_death_gate : public SpellScriptLoader
             void Register()
             {
                 OnCheckCast += SpellCheckCastFn(spell_dk_death_gate_SpellScript::CheckClass);
-                OnEffect += SpellEffectFn(spell_dk_death_gate_SpellScript::HandleScript, EFFECT_0, SPELL_EFFECT_SCRIPT_EFFECT);
+                OnEffectHitTarget += SpellEffectFn(spell_dk_death_gate_SpellScript::HandleScript, EFFECT_0, SPELL_EFFECT_SCRIPT_EFFECT);
             }
         };
 
@@ -393,7 +393,7 @@ class spell_dk_scourge_strike : public SpellScriptLoader
 
             void Register()
             {
-                OnEffect += SpellEffectFn(spell_dk_scourge_strike_SpellScript::HandleDummy, EFFECT_2, SPELL_EFFECT_DUMMY);
+                OnEffectHitTarget += SpellEffectFn(spell_dk_scourge_strike_SpellScript::HandleDummy, EFFECT_2, SPELL_EFFECT_DUMMY);
             }
         };