(bug 3173) Option to offer exported pages as a download, rather than displaying inlin...

(bug 3173) Option to offer exported pages as a download, rather than displaying inline, as in most browsers; set Content-disposition: attachment with a filename incorporating the site name and timestamp. Defaults to being checked. Also rewrote the Export form generator to use Xml class methods; tweaked spacing on the existing check box. Tweaked English default for "exporttext".