Optionally hide the "List" checkbox

There may be situations where you want all your users' files to be displayed in any case, so that the user has got no "List" option displayed at all. (In which case "List" has to be set to enabled of course, not displaying any files at all can still be chosen in the "Display fields" section of each content type.)

So this patch provides an additional checkbox in the widget settings, called 'Show "List" option'. It depends on the themed output patch though, because that's what it modifies in order to hide the "List" checkbox.