Also accept callables for the 'style' arguments for containers. Removed 'focussed' class in 'Window'.
Also accept callables for the 'style' arguments for containers. Removed 'focussed' class in 'Window'.

For the styling, the 'focus' concept usually doesn't apply on a single
'Window', but on a set of containers. Adding this by default on the
`Window` doesn't make sense. If needed, we better create a helper
container to insert the 'focus' style to all its children if one of the
children is focussed.

jonathanslenders/python-prompt-toolkit