org-export: Define tools for tables, table rows and table cells

org-export: Define tools for tables, table rows and table cells

* contrib/lisp/org-export.el (org-export-table-cell-width,
  org-export-table-cell-alignment, org-export-table-cell-borders,
  org-export-table-row-group, org-export-table-has-special-column-p,
  org-export-table-row-is-special-p, org-export-get-parent-table,
  org-export-table-dimensions, org-export-table-cell-address,
  org-export-get-table-cell-at, org-export-table-has-header-p,
  org-export-table-cell-starts-colgroup-p,
  org-export-table-cell-ends-colgroup-p,
  org-export-table-row-starts-rowgroup-p,
  org-export-table-row-ends-rowgroup-p,
  org-export-table-row-starts-header-p,
  org-export-table-row-ends-header-p): New functions.
(org-export-table-format-info, org-export-clean-table): Removed
functions.
(org-export-filter-table-cell-functions,
org-export-filter-table-row-functions): New variables.
(org-export-filters-alist): Install new filters.
(org-export-collect-tree-properties, org-export--skip-p): Mark special
rows and cells as ignored.
* testing/lisp/test-org-export.el: Add tests.