Add the ability to filter the list of games in the popup dialog

I feel like this would be a great addition for people who have hundreds of items on Steam. It would help immensely to be able to quickly filter the list that shows up in the popup.

I don't have any experience with IUP myself, or else I'd love to help. Possibly, something like this [IUP Mask](https://www.tecgraf.puc-rio.br/iup/en/attrib/iup_mask.html) could be used?