add possibility to dump all scanner option properties into a text file

This adds the possibility to dump all the information we have about the options of a scanner into a simple text file.

Lots of times, I am unable to debug bug reports (e.g. here) since users have different hardware with a different SANE backend. This is an attempt to get at least an idea what is happening. I don't know how useful it's going to be in reality.

A simple implementation for skanpage is here

An exemplary output file is attached options.txt