Fix: with --new-widgets advanced preferences may not shrink on deselect

Partial relayout does not make a grid yield more space to its parent grid.
Thus, only firing partial relayout as a result of hiding a widget is *not*
safe.

Fixed by triggering full relayout instead, like the code already does
without --new-widgets.

src/gui/widgets/widget.cpp

@@ -489,7 +489,7 @@ void widget::set_visible(const visibility visible)
 	visible_ = visible;
 
 	if(need_resize) {
-		if(new_widgets) {
+		if(visible == visibility::visible && new_widgets) {
 			event::message message;
 			fire(event::REQUEST_PLACEMENT, *this, message);
 		} else {