notebook: edit, rerun, delete, hide, clone... actions on user-code blocks

[switham]

A few action buttons, or at least actions, available on each code block in the notebook:

Edit (implicit), rerun (run becomes rerun once you have run once), interrupt or "stop" while one is running, delete, hide/shrink, clone. Insert an empty block. Delete a block (and its output). (Insert + delete + copy + paste give you merge and split.)

Maybe: save as text file, load from text file?