Addding a method to indicate the end of a component initialisation phase (used in Model.load and Model.create_component)
Adding a method that dump component attributes (used in Model.create_component and Model.delete_component to call the migration_handler)

No extended checks was done. Only tested if the check passes without syntax error and without failing with no reason

At creation if no rank specified the component will be place last, else you can pass an integer representing an absolute rank or two specials strings 'last' or 'first'

EmComponent::check() always return True but rearrange ranks if they are not continuous

comment selected_fields because it clashes with ._fields[]
 => we have to rethink of ._fields[] and fieldtypes

Now Model as an attribute named migration_handler
DummyMigrationHandler has only one method : register_change

Model._components now contains only one dict ( 'uids' key ) and the others are now list of EmComponent instances
Model now handle EVERY access to the _components dict

There is now two methods Model.name_from_emclass and emclass_from_name that allows to access the Model.components array