Allow setting the ID of the main table in HTMLForm

Allow setting the ID of the main table in HTMLForm

- Added new method HTMLForm::setTableId() to allow to set the ID
  of the main table (or the outermost <div> element depending on
  the format).
- Changed the second parameter of HTMLForm::displaySection()
  to take the full ID so that the one set with setTableId()
  will not get prefixed with "mw-htmlform-"

Note: If someone has a better idea for the name of that method
(since it also applies to <div> elements), please tell me it :)

Change-Id: Id01bd0fa44512b0320f3c3ff79627e8a9bccd5bb