bug#71386: 29.1; Frame is auto-deleted even when it has multiple tabs

[martin rudalics]

> Sorry, I don't understand what I should do in tab-bar-mode
> for quit-restore-window.  I expected that changes should be
> in window.el.

Provide an option, say 'tab-bar-save-frame', that allows users to
customize whether a frame should be deleted when "other tabs are
available for that frame".  And provide a function, say
'tab-bar-save-frame-p', 'window--delete' could call thusly

    (let ((deletable (window-deletable-p window)))
      (cond
       ((eq deletable 'frame)
        (let ((frame (window-frame window)))
          (cond
           ((and (fboundp 'tab-bar-save-frame-p)
                 (tab-bar-save-frame-p frame kill))
            nil)
           (kill
            (delete-frame frame))

thus avoiding to kill the frame when that function returns non-nil.
'tab-bar-save-frame-p' itself would be free to do with the frame
whatever it wants according to the value of 'tab-bar-save-frame'.

martin