Create PDF Renderer

Implement a PDF Renderer based on iText to directly produce PDF based on a XDOM.

Note that we might also need another PDF implementation which would take some XDOM, generate XHTML, use CSS4J to apply styles on it and then use XSL-FO and Apache FOP to generate the PDF. Whether we need this other renderer or not will depend on how easily we can implement a solution to "style" a XDOM.

The advantage of a direct PDF generation approach s that it reduces the conversion layers from at least 4 (XDOM to XHTML, then CSS4J, then XHTML to XSL-FO, then XSL-FO to PDF) down to 1. The less conversion layers the less likely bugs will appear...

The cons of a direct PDF generation approach is that it doesn't support handling RawBlocks (HTML macro for example generates RawBlocks). For support those we need to transform HTML in XSL-FO and then in PDF, with the issues that applying styles on HTML is probably not the same as applying styles on an XDOM tree. A custom solution could be to introduce an UnknownBlock and have RawBlock being parsed by the PDF Renderer when the syntax is HTML and generate UnknownBlock for unknown blocks (such as Form blocks).