Added XmlText Preconverter to handle internal links.

See https://jira.ez.no/browse/EZP-20533

This pull adds a pre-processing step to the XMLText converter to handle node_id and object_id attributes on \<link /\> ezxml tags. 

Currently, the [xslt](http://www.teamusa.org/~/media/TeamUSA/Gymnastics/Maroney_McKayla_vaultfinal_300x375_2012.jpg) only considers the 'url' attribute of the link element in an xmltext field. This means that node_id and object_id attributes are ignored, creating empty A tags when xmltext fields are processed via the non-legacy stack (eg via ez_render_field from a Twig template). 

The preprocessor scans the document for link tags, and populates the url attribute for any link tags with node_id or object_id attributes.