Edit sections after data entry

[grahamkirby]

Sometimes I realise part-way through marking that a section is redundant, or that one is missing.

[mtorpey]

We're part of the way into this, in that section headings can now be modified (the title changed). I guess this is completing the picture with adding, removing and reordering sections.

Things to consider:

• What happens when you delete a section with some contents? (data loss warning?)
• What's the best UI for this? Adding more controls to the main window increases rarely-used clutter and probably risks accidents. A menu option like "Edit sections" is probably more appropriate, and then we could remove the ✏️ symbols from the main screen.
• There should probably be a unified approach for changing settings after starting an assignment. We've bolted these on one at a time (add students, change headings, ...) but maybe it should be possible to simply reopen the configuration window and adjust it using the same interface used for setup (presumably with some locked items).