Support resuming from journal

[chimosky]

When activity is resumed from the journal, last activity state is saved and hence last activity state is lost.

Implement read_file and write_file methods to enable support for resuming from journal.

[quozl]

This is said to be a prototype activity, and I don't think it is ready for use. I think the workflow needs to be designed again, as it was originally written when Sugar was quite different. https://help.sugarlabs.org/en/info_slicer.html holds the current documentation; it was moved in 2018.