Notebook Actions second part.

On top of #6668 

Start using actions for toolbar, and other stuff.

In particular, toolbar can be "just" described as a `[list of groups]`. and each group is a `[list of action name]`.
technically a group is either a list of action name or a specific value for something that cannot be done as an action (typically dropdown to select cell type), but that's only for private use. 

Also in this PR:
  - Render shortcut nicely on mac with unicode value of Cmd, Esc, Up Down, ... etc. 
  - Do not use dom ('#cell_type') to link things together, so technically you can have many toolbar with many dropdown that allow to select cell type, and everything works.
  - Use relative import in require.

 
These look like notes to self, rather than comments that should be kept. Especially the second one, since it looks like the group name is second and optional.