Add 1-1 mapping between windows and buffers

[Ambrevar]

As suggest by /u/kvtb in https://old.reddit.com/r/next_browser/comments/e969nu/using_next_within_emacs_with_exwm_workflow/:

It would be nice to have a mode which locks a specific buffer to a specific window, or in other words, creates a 1-1 mapping between buffers and windows.

This would be useful to let the window managers manager the buffers, e.g. what EXWM can do with Qutebrowser when the option "tab is window" is on.

[Ambrevar]

Also see #854 for a discussion on the new buffers structure. It could be a good opportunity to ensure that we can map windows to buffers.

[aadcg]

Personally, I'd be really interested in the feature, but this should be developed as an extension first.

[aadcg]

On second thoughts, this is a must-have feature.

[jmercouris]

Not planned.