Made xp sharing less ugly by reworking McMMOPlayer xp methods

Notable consequence: checkXP and addLevel in ExperienceAPI are no longer
needed and became deprecated

src/main/java/com/gmail/nossr50/skills/woodcutting/TreeFeller.java

@@ -220,6 +220,6 @@ public final class TreeFeller {
             block.setType(Material.AIR);
         }
 
-        mcMMOPlayer.addXp(SkillType.WOODCUTTING, xp);
+        mcMMOPlayer.beginXpGain(SkillType.WOODCUTTING, xp);
     }
 }