fix : reduce piskel model size : ImportController

+ ImportController is no longer relying on the deserializer to build a
  piskel instance
+ Static builders have been added to Piskel and Layer to help easily
  create new instances from existing elements
fix : reduce piskel model size

+ moved Serializer and Deserializer to utils.serialization package
+ put all backward code in utils.serialization.backward
+ added static method on Deserializer to make its usage similar to other
  utils in the package
- still not happy with the names used in Deserializer classes
  (deserializer.deserialize ...)