org-odt.el: Put label generation and reference under user control

org-odt.el: Put label generation and reference under user control

* contrib/lisp/org-odt.el (org-odt-begin-table)
(org-export-odt-format-formula, org-export-odt-format-image)
(org-odt-entity-frame-styles, org-odt-format-entity): Use
"__Table__", "__Figure__", "__MathFormula__",
"__DvipngImage__" as internal category handles.
(org-export-odt-user-categories)
(org-export-odt-get-category-from-label)
(org-odt-label-styles, org-odt-category-map-alist): New
variables.
(org-odt-entity-labels-alist): Add label style as an
additional entry.  Update docstring.
(org-odt-get-label-category-and-style): New defun.
(org-odt-add-label-definition)
(org-odt-format-label-definition)
(org-odt-format-label-reference)
(org-odt-fixup-label-references)
(org-odt-format-entity-caption, org-odt-init-outfile):
Modified.
(org-odt-label-def-ref-spec): Removed.  Superceded by
`org-odt-label-styles'.
(org-odt-get-label-definition): Removed

Give user more control over how labels are generated and
referenced.  This is accomplished by mapping each label to a
category-handle which in turn is used to locate the category,
counter and style to be associated with label.