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.
... because ```$( ".filterable-select" )``` contains multiple elements and we need to check the id of each, not just the first.

Do you think the following alternative is more legible?
```JS
var dialogBelongsToSelectmenu = false;

$( ".filterable-select" ).each( function() {
  if ( $( this ).attr( "id" ) + "-dialog" === id ) {
    dialogBelongsToSelectmenu = true;
    return false;
  }
});

if ( dialogBelongsToSelectmenu ) {
```