ability to save/restore full viewer state

[gdementen]

As a followup to #88. Users would like to be able to save the full state of the viewer, and restore it later.

This could also help for debugging users problems: "send me your session" would be easier to tell them than "please create a small example problem reproducing the problem".

[gdementen]

this is mostly #153 plus the ipython history + set the current prompt number + the current traceback, if any.

[gdementen]

for the traceback, this could help: https://github.com/ionelmc/python-tblib