XWikiDocument.getDisplayTitle() should be evaluated to support Velocity/Groovy code in titles

When a document has no title, the method extractTitle tries to get the title from the content.

Example:

• In my document :
  1 $msg.get("myTitle")
• In my bundle:
  myTitle=Welcome to XWiki

$doc.displayTitle (which calls extractTitle()) should return
Welcome to XWiki
but returns
$msg.get("myTitle")