Export ledger values to spreadsheet. This is useful in markets
that require extensive customization of illustrations. It can be
done today, but only as a side effect that was tacked on to
printing in a great hurry:
(1) the steps are unintuitive
 - type 'idiosyncrasy_spreadsheet' in 'Comments'
 - run illustration
 - Print | preview [close preview: want only the side effect]
(2) the result shows the lack of deliberate design
 - the file always has the same name
 - older file instances are overwritten
 - if the file is already open, then no new file is written
(3) any change in file layout requires changes in downstream
spreadsheets that depend upon it.

Analysis:

(1) The file is created in
  void Ledger::write(xml::node& x) const
which probably should be refactored as part of this task
  http://savannah.nongnu.org/support/?105591
anyway. This facility may be inappropriate for some users, so it
is probably best to place it on a new home-office-only menu.

(2) Existing function
  unique_filepath()
could be used to create the file with a unique name.

(3) Users should be encouraged to use range names in spreadsheets
they create, to insulate themselves from changes. This file is a
simple tab-delimited representation of the 'ledger' object, with
column headers that are suitable for forming range names. When a
new column is added, it should be available in this file.