Selectmenu: Filterable demo now filters/refreshes selectmenu widget

Fixes gh-7677

... and actually fixes a bug in the existing selectmenu-custom-filter demo whereby the filter input does not return to the popup if the menu is ever shown as a dialog.
Another alternative is to keep the check but improve the comment. Something like
```JS
// Check all the select menus to see if the id of the dialog matches the id of one of them, because
// we only handle the appearance of a dialog generated by a filterable selectmenu
```