[dev-context] enabling single column page view

Hans, it would be extremely useful to have the singlesided value from the option key in \setupinteractionscreen that allows single column page. The attached diff implements this. It allows the page to be displayed in full with. Of course, fit (/FitView) will kill the gain of enabling single column page view to allow full width display. This minimal sample displays the usage (once the patch is applied): \setupinteraction[state=start] \setupinteractionscreen[option={singlesided}] \starttext a \stoptext Could you consider adding it to next release? Many thanks for your help, Pablo