Fix the session file on disk

[brichet]

Currently, we have two slightly different session file structures.
The regular session file (from glue core) has be intentionally simplified, as the frontend do not need all its information, and has to increment it itself.
It results in a file (on disk) which can be sometime corrupted and unusable by glue QT application.

One option could be to avoid modifying the session file content in the frontend, and to use the kernel with its serializer for all the modifications in the app (as was done for adding data).

NOTES

A problem to be solved for this operation is the serialization of the bqplot viewer, which doesn't work with the GlueSerializer.