Refactor footnotes to have autonumber markup

[opoudjis]

Child of #610:

We will replace the body of fn with fmt-fn-body, which will contain the footnote body with autonumbering of footnotes, instead of expecting HTML and PDF to insert that autonumbering in the footnote body. This is to deal with the fact that JIS inserts the word "footnote" before footnote text.

So currently inside a table:

 <td align="center">
    Balilla
    <fn reference="a)">
      <p id="_">Parboiled rice.</p>
    </fn>
</td>

Renders in JIS as:

<td scope="col">
                   Balilla
                   <a href="#tableD-1a)" class="TableFootnoteRef">a)</a>
                 </td>

in the table, and

<div class="TableFootnote">
                     <div id="fn:tableD-1a)">
                       <p id="_" class="TableFootnote">
                         <span>
                           Footnote
                           <span id="tableD-1a)" class="TableFootnoteRef">a)</span>
                            <tab/>
                         </span>
                         Parboiled rice.
                       </p>
                     </div>
                   </div>

It is not HTML and PDF's job to insert the word "Footnote" before the "a)" and a tab after it in the footnote body, it is Presentation XML's. So the Presentation XML will change to:

 <td align="center">
    Balilla
    <fn reference="a)" id="{ADDED ID}">
      <p id="_">Parboiled rice.</p> 
      <!-- ignore all content of fn but fmt-fn-body -->
      <fmt-fn-body>
        <p id="{new GUID}">
          <span class="fmt-caption-label">
           <!-- most flavours do not add "Footnote" -->
            <span class="fmt-element-name">Footnote</span>
             <semx element="autonum" source="{ADDED ID}">a</semx>
            <span class="fmt-autonum-delim">)</span>
          </span>
          <span class-"fmt-caption-delim"><tab/></span>
          Parboiled rice.
        </p>
      </fmt-fn-body>
    </fn>
</td>

Added bonus: the ADDED ID GUID allows more straightforward implementation of backlinks from footnote text.