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.
Since I perform this check twice, if you think the above alternative is more palatable, then I will write a function that contains it and that gets called from both places. Something like ```isSelectmenuDialog()```.