Move stuff into commons, rename PermissionObject, add more javadocs to api

api/src/main/java/me/lucko/luckperms/exceptions/ObjectAlreadyHasException.java

@@ -1,4 +1,8 @@
 package me.lucko.luckperms.exceptions;
 
+/**
+ * Thrown when a permission holding object already has a permission, is already a member of a group, or when a track
+ * already contains a group.
+ */
 public class ObjectAlreadyHasException extends Exception {
 }

api/src/main/java/me/lucko/luckperms/exceptions/ObjectLacksException.java

@@ -1,4 +1,8 @@
 package me.lucko.luckperms.exceptions;
 
+/**
+ * Thrown when a permission holding object does not already have a permission, is not already a member of a group,
+ * or when a track doesn't contain a group.
+ */
 public class ObjectLacksException extends Exception {
 }