Updated PDF conversion help

I have a reworked version of reflow.py for the new PDF engine.
It seems to work better than the standard method, so I would like to publish it so that others can add improvements.

Before doing that, I need to update the Preferences settings to add new fields to assist the conversion. I have not been able to find the correct method for doing this.

I know the UI fields to set up, but I have not found out where/how to save these and pass them to the input.

I have not tested it with ebook-convert. I assume that also needs new command-line options.

Any help would be much appreciated.

Alan