org-odt: Various improvements to table and image references.

org-odt: Various improvements to table and image references.

* lisp/org-odt.el (org-odt-parse-label): New utility
routine. Parses labels in to it's component parts the entity
catgory (like Figure, Illustration etc) and it's sequence
number.
(org-export-odt-preprocess): Use org-odt-parse-label. Sequence
references now use "category and value" setting. Emit the
place holder text in the new format..
(org-odt-default-entity): New variable. The default entity to
which an image or table belongs when the label leaves it
unspecified.
(org-odt-format-entity-caption) : Use org-odt-parse-label and
org-odt-default-enity. Also re-defined the param list.
(org-odt-begin-table, org-export-odt-do-format-image):
Modified to accomodate the changes in signature of
org-odt-format-entity-caption.