[calib2] Returns the application state between 2 uses

[vallsv]

Store the state of the application when it is close. To be able to retrieve it the next time.

• It could be quite easy to dump the application modelization into a file.
• Then it could be relativly easy to reach back this data to regenerate the previous modelization.
• The file should be stored in the user account / maybe .config or .cache

[kif]

1. Serialize the pyFAI.gui.model.CalibrationModel + composed objects.
2. restore are boot-up time

Issue: what if the user provides a config online vs the last stored state