#1 Map fields handling

[Luigi Dell'Aquila]

Dejavu should manage Map fields: it should be possible to declare for the elements in a Map field to be saved and restored.

For example,
- an object has a field that is an instance of Map and contains one element

- the object is retained

- another elements is added to the map

- the object is recalled

- the map should now contain only the element it contained at the first retain.