Fix some compilation problems

The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.getPrimaryGroup()
The type xOfflinePermissionUser must implement the inherited abstract
method MetaDataHolder.getMetaData(String)
The type xOfflinePermissionUser must implement the inherited abstract
method MetaDataHolder.setMetaData(MetaData)
The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.isMember(String)
The type xOfflinePermissionUser must implement the inherited abstract
method MetaDataHolder.hasMetaData(String)
The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.setMember(PermissionGroup, boolean)
The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.removePermission(String)
The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.getGroups()
The type xPermissionManager must implement the inherited abstract method
PermissionManager.getGroup(String)
The type xOfflinePermissionUser must implement the inherited abstract
method OfflinePermissionUser.addPermission(String)
We need only one PermissionManager!