Explore moving notebook tools into core

[DonJayamanne]

• Drawbacks?
• Benefits?
• Does this need careful planning?

[amunger]

the only benefit that comes to mind is better access to whether a cell is currently executing. I think we have an OK solution for that as-is tho.
Custom UI is also a lot easier to implement, but I don't have any specific thoughts for what we would want there.