[various] update add_buttons_group to use action instead of button

[jcb91]

As discussed in #1102

[jcb91]

This looks mainly ok, but a few changes are needed:

1. rebase onto current master, as there are now some merge conflicts
2. accessing Jupyter.actions raises warnings, so it seems better to do it through Jupyter.keyboard_manager.actions.
3. whenever we call ActionHandler.register, we should provide both an action name and a prefix (the prefix should be the nbextension filename, similar to that used in logs for lots of examples)
4. various buttons were being created with ids, which the new formulation seems to have used as action names. This is ok, but they (the actions) need to have an id key set as well

[Sukneet]

Are you sure actions have an id key? I can't see that documented anywhere.
https://github.com/jupyter/notebook/blob/master/notebook/static/notebook/js/actions.js

Although, it won't hurt to add I guess.

[jcb91]

Apologies, you're right, the action doesn't have an id, so instead we need to pass an object with keys id and action to the toolbar registration function 😲

[Sukneet]

Sorry for not getting to this! Been pretty busy. I was going to do it tomorrow, but looks like you already did it. :)