org-odt.el: Improve how labels are applied and generated.

org-odt.el: Improve how labels are applied and generated.

* contrib/lisp/org-odt.el
(org-odt-entity-labels-alist, org-odt-entity-counts-plist)
(org-odt-label-def-ref-spec): New variables.
(org-odt-init-outfile): Reset above variables.
(org-odt-add-label-definition, org-odt-get-label-definition)
(org-odt-format-label-definition)
(org-odt-format-label-reference)
(org-odt-fixup-label-references): New functions.
(org-export-odt-preprocess-label-references): Modified.
(org-odt-end-export): Call `org-odt-fixup-label-references'.
(org-odt-parse-label): Removed
(org-odt-default-entity): Removed.
(org-odt-format-entity-caption): Fixed signature so that
category param is no longer optional.  Use
`org-odt-format-label-definition'.
(org-export-odt-do-format-formula)
(org-export-odt-do-format-image): Pass the mandatory category
param to `org-odt-format-entity-caption'.

* contrib/odt/styles/OrgOdtContentTemplate.xml: Add Figure and
Equation as sequence declarations.

Most of these changes can be pushed to org-exp.el.  For now park the
changes locally to org-odt.el.