Add XP_GAIN combat text type; show '+N XP' in purple on kills

XP gain was previously shown as a HEAL entry (green +N) which conflates
it with actual healing.  New XP_GAIN type renders as purple '+N XP' in the
outgoing column, matching WoW's floating XP style.

include/game/spell_defines.hpp

@@ -51,7 +51,7 @@ struct CombatTextEntry {
     enum Type : uint8_t {
         MELEE_DAMAGE, SPELL_DAMAGE, HEAL, MISS, DODGE, PARRY, BLOCK,
         CRIT_DAMAGE, CRIT_HEAL, PERIODIC_DAMAGE, PERIODIC_HEAL, ENVIRONMENTAL,
-        ENERGIZE
+        ENERGIZE, XP_GAIN
     };
     Type type;
     int32_t amount = 0;