update quickhelp on adding and removing shortcuts

I've been knee deep in making shortcuts work for vim mode etc, and have more PRs coming to start some discussion about that, but this one seems like a separate small change worth having.

Right now, we have an API for adding and removing shortcuts, but no visual indication in the UI of which shortcuts are actually active (the quick help you see in the notebook is built only once, and reused there-after).

With this change, we just rebuild the quickhelp dialog on keyboard shortcut changes