- Collect all shell modules into a new top-level 'modules' directory:

      $(top_srcdir)/modules/addressbook
      $(top_srcdir)/modules/calendar
      $(top_srcdir)/modules/mail

  Nothing is allowed to link to these, not plugins nor other modules.

  THIS SOLVES BUG #571275 AND OPENS THE DOOR TO PORTING TO MAC OS X.

- Mimic the libevolution-mail-shared library from master (except drop
  the "shared" suffix) and have libevolution-mail-importers and all
  mail-related plugins link to it.

- Discard the a11y subdirectories and have the files live alongside
  their counterpart widgets.