[RFC] Use QWeb to generate legal document XML

[dreispt]

Currently Jinja2 is being used.
QWeb has the advantage of allowing inheritance, making for option available for customer or sector specific enhancements.
There is a report_xml module enabling this, used in the Spanish localization for the same purpose.

Initially, we could have the rendering engine selectable, between Jinja and QWeb, to not be forced to redesign the legal document right away.

[pedrobaeza]

You can check in https://github.com/OCA/l10n-spain/blob/12.0/l10n_es_facturae/views/report_facturae.xml that is very simply to provide QWeb XML templates with report_xml