Background tasks

[brichet]

This PR allows performing some actions in background, without opening the widget.

Currently the actions that can be performed in the background are:

• notebook: add cell
• notebook: delete cell
• notebook: set cell content
• notebook: get cell/notebook info

output.webm

Fixes #4

Code change

It adds a background argument to the commands. If true, the tasks create a notebook context instead of opening the widget. When the task is over, the file is saved.
If a widget is already opened, the task doesn't run in the background, it runs in the widget directly, as before.

TODO

• add a backup of the file, to avoid loosing some content when a task run in background.