Expose cell ID in the UI

[krassowski]

This is aimed to help with jupyter#639. The cell ID will now be shown above the modified cells:

[krassowski]

Hi @vidartf just checking if you saw this one. Any thoughts?

[vidartf]

Sorry I missed this! Thanks for the help! I think it looks good in principle, but these thoughts:

• The way I read this, it only shows the cell IDs if they have changed.
• If it is only shown when changed (which is a rare occurrence), it will be rare for users to see them. Will they then:
  • Know what it means?
  • Notice them?

Potential ways to address above points:

• Color highlight them? It might be hard to get this right unless we highlight the entire header, and that might be confusing.
• Use a stronger font color (the current color2 is rather unimposing, while we might wish to draw more attention to it).
• Have a label "Cell ID", or tooltip "Cell ID changed"? Something indicating to the user what this weird string of random characters are, and maybe why it is a big deal?