org-element: Internally store headline/inlinetask's tags as a list of strings

org-element: Internally store headline/inlinetask's tags as a list of strings

* contrib/lisp/org-element.el (org-element-headline-parser,
  org-element-headline-interpreter, org-element-inlinetask-parser,
  org-element-inlinetask-interpreter): Store headline/inlinetask's
  tags as a list of strings.
* contrib/lisp/org-export.el (org-export--selected-trees,
  org-export--skip-p): Use new tag representation.
* contrib/lisp/org-e-ascii.el (org-e-ascii--build-title,
  org-e-ascii-format-inlinetask-function, org-e-ascii-inlinetask): Use
  new tag representation.
* contrib/lisp/org-e-html.el (org-e-html-format-headline-function,
  org-e-html-format-inlinetask-function, org-e-html--tags): Use new
  tag representation.
* contrib/lisp/org-e-latex.el (org-e-latex-format-headline-function,
  org-e-latex-format-inlinetask-function, org-e-latex-headline,
  org-e-latex-inlinetask): Use new tag representation.
* contrib/lisp/org-e-odt.el (org-e-odt-format-org-tags): Use new tag
  representation.
* testing/lisp/test-org-element.el: Update test.