org-odt.el: Make category component of captions configurable

org-odt.el: Make category component of captions configurable

* lisp/org-odt.el (org-export-odt-category-strings): New
custom variable.
(org-odt-category-map-alist): Modify interpretation.  Don't
use the same field to double up as both a OpenDocument
variable and a category string.  Entries in this list now
specify only the OpenDocument variable.  Category strings are
obtained through an indirect lookup of
`org-export-odt-category-strings'.  Use same OpenDocument
variables as what LibreOffice uses for various entities.  Fix
docstring.
(org-odt-add-label-definition)
(org-odt-format-label-definition)
(org-odt-format-label-reference): Propagate above changes.

Fix for the following bug:
http://lists.gnu.org/archive/html/emacs-orgmode/2012-03/msg00628.html