XWIKI-9923: The generated Table of Contents is empty for the ODT export
* officeExport.xsl was expecting the content directly under xwikimaincontainer DIV, but the content is under xwikicontent DIV.
* remove duplicated code in pdf.vm and make sure the xwikicontent DIV is generated both when a single page is exported and when multiple ones are.

(cherry picked from commit ba4cebced6a9426f92a99f65880ab357bdb3b816)
XWIKI-9923: The generated Table of Contents is empty for the ODT export
* Update xpath that matches headings in xhtml2fo.xsl

(cherry picked from commit 68ba3c0a2ce82029fc2b277b74166b206be96d8b)