Try to address issue reported by Michael Aram: export variables sooner, when the uplevel they come from is still well known, rather than later in the rendering, when this could not be so trivial (e.g. in OO code)
ad_form: allow multiple files in 'template::widget::file' by using the 'multiple' attribute of the input tag, new on HTML5. - Pass the 'multiple' flag to 'template::element::create', so it can be used by widgets to modify their behavior. - Update 'template::element::create' doc. - This opens the door to reducing the number of widgets by combining single and multivalued ones into one (for instance, 'multiselect' and 'select' widgets), in the same way the 'file' widget does now.