[emacs-w3m:13389] w3m-bookmark-add-all-urls

Emanuel Berg's trouble with bookmarks [13385] got me looking at some of
the code, and while I can't reproduce his bug report or find a related
problem in the code, I do see a related function that could do with some
changes.

Function `w3m-bookmark-add-all-urls' is supposed to add a bookmark for
all currently visited pages, ie. all current buffers, ie. all current
tabs.

1] The function is only being performed when variable `w3m-use-tab' is
   non-nil. This is pretty clearly a bug because there are
   `w3m-display-mode's that don't use visual tabs, but do support
   multiple buffers.

2] For each and every open buffer, the function currently separately
   prompts the user for a 'bookmark section' and a 'bookmark title',
   which can quickly become mind-numbingly monotonous. Moreso since I
   imagine the use-case for the function is help a lazy person who wants
   to quickly bookmark 'everything', and 'everything' is sometimes for
   me > 100 buffers.

   The resolution for this might not be as clear-cut as for item #1,
   which is why I'm posting this instead of just writing code and making
   a pull-request. What seems to me to be a reasonable solution is to
   prompt the user once and only once for a 'bookmark section', with the
   default being "Saved bookmarks YYYY-MM-DD HH:MM:SS", and not
   prompting for bookmark titles, ie. use the default values. This gets
   the job done quickly, and the user can at any future point edit the
   bookmark file.

   Feedback is welcome, please.

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