[session] provide a simple transaction context manager for session. Closes #1725640

[formwidgets] make the select size a class attribute (more easily tuned)

[bfss] Make ``current_fs_path`` return None when an attribute has no value current_fs_path was returning the future fspath when no value was defined. * This does not make any sense * This tricks the storage into believing that updating the attribute content is not necessary. The file was written but the attributes was not updated. as a result the assignement was not made. (close #1725446)