Allow step edition

When editing a sequence, only the last step is editable. If you made a mistake, you have to remove all the steps until you can edit the faulty one.
It would be nice to select which one to edit.

The issue names no files, tests, or entry points. Start by locating the sequence-editing interface and tracing how the last step becomes editable. Define how selecting an earlier step should work and verify that a mistaken step can be edited without removing later steps.

Written by the indexing model from the issue text.