Added AVT support to navindex attribute

[tjoris]

A proposed implementation for this question because we wanted to use navindexes in repeats as well.
http://discuss.orbeon.com/Tabindex-in-repeating-region-help-td44715.html

Tabindex attributes are still copied to the HTML output (if there is no navindex attribute), but do not support AVTs.

This patch probably needs some cleanup and rework:

1. src/main/scala/org/orbeon/oxf/xforms/control/ControlExtensionAttributesSupport.scala
   => The method name addExtensionAttributesExceptClassAndAcceptForHandler doesn't reflect its behavior anymore. The extra check to exclude the navindex attribute was needed, because otherwise the control's span element received a navindex attribute. It needs to processed when sending an ajax response however (in addExtensionAttributesExceptClassAndAcceptForAjax)
2. src/resources-packaged/ops/javascript/orbeon/xforms/server/AjaxServer.js
   => Not sure if this is the way to change the tabindex attribute of a child. We couldn't find another example of an ajax response that set an attribute on a child HTML-element of the control. We also tried to specify the ID of the child-element in the server, but couldn't find an example of that either. This code also does the translation from navindex to tabindex in Javascript, which might not be desirable as well.

[ebruchez]

Thanks for this, I plan to review the changes.