Add a file switcher instance to this plugin

[ccordoba12]

It'd be really nice to have it to switch among notebooks.

@goanpeca, question: Ctrl+P (the shortcut to call the switcher) is a global one right now, but I think we should use the same one here. So what do you think we should do to make that work?

Possibilities:

1. Make Ctrl+P a plugin shortcut. Cons: Not nice because you'd have to give focus to the Editor or the notebook to make it work.
2. Make Ctrl+P to detect the current plugin and change its behavior accordingly.
3. Add an API for the file switcher so external plugins can register with it.

[goanpeca]

Well probably 3, I guess? The question is, should this display notebooks only or both editor files and notebooks?

[ccordoba12]

I think notebooks. It'd be less confusing, right?

[goanpeca]

Sure 👍