Propose alternative templates for PDF exports

This lack of functionality was first reported on this forum discussion .

The conclusion we arrived at is that we should have at least two templates to chose from by default for PDF export: one which is pretty much just WYSIWYG, the other one which is black/white and adapted to print correctly.

There might be a need to keep the current template as an intermediate option: most things come from the colortheme but a few properties are changed to try and get something correct to print (namely, the main section background is set to white).