Annotate Dvipng images/Math formula and strengthen error handling

Annotate Dvipng images/Math formula and strengthen error handling

* contrib/lisp/org-odt.el (org-odt-encode-plain-text): New.
(org-odt-format-line)
(org-odt-format-source-code-or-example-plain): Use it.
(org-odt-format-object-description): New.  Format svg:title
and svg:desc elements out of plain text.
(org-export-odt-format-formula, org-export-odt-format-image):
Check whether the incoming formula or image is generated out
of a latex fragment.  If yes, pass the latex fragment for
downstream processing.
(org-odt-format-frame): Check if the object that is enclosed
in the frame has metadata associated with it.  If yes, dump it
as part of svg:title and svg:desc elements.
(org-odt-protect-latex-fragment): New after advice for
`org-format-latex-as-mathml'.  Fixes the following bug
http://lists.gnu.org/archive/html/emacs-orgmode/2011-11/msg00176.html