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/World/boss_emerald_dragons.cpp

@@ -306,7 +306,7 @@ class spell_mark_of_nature : public SpellScriptLoader
             void Register()
             {
                 OnUnitTargetSelect += SpellUnitTargetFn(spell_mark_of_nature_SpellScript::FilterTargets, EFFECT_0, TARGET_UNIT_SRC_AREA_ENEMY);
-                OnEffect += SpellEffectFn(spell_mark_of_nature_SpellScript::HandleEffect, EFFECT_0, SPELL_EFFECT_APPLY_AURA);
+                OnEffectHitTarget += SpellEffectFn(spell_mark_of_nature_SpellScript::HandleEffect, EFFECT_0, SPELL_EFFECT_APPLY_AURA);
             }
         };