Upstream improvements

[stsewd]

This issue is to track improvements we can send upstream (python-prompt-toolkit specially)

• The list widget Create custom list/tree widget #91 List of buttons, instead of list of check or radio buttons? prompt-toolkit/python-prompt-toolkit#1125
• Helper to replace children of an already created container
• Improve docs around more complex application
• Document all the widgets

[stsewd]

I have sent some https://github.com/prompt-toolkit/python-prompt-toolkit/pulls/stsewd already :)

[stsewd]

Another improvement we could send upstream #133