Modify missing icon entry for a different icon

Line 39 of the app-select python application contains: missing_icon = "gtk-missing-image" When using basic Adwaita theme, gtk-missing-image doesn't seem to exist, so app-select cannot launch. As apps found by app-select should "launch something", I propose using application-x-executable icon instead. Line 39 would look like: missing_icon = "application-x-executable"

I think this change should work for all icon themes.

Hope this helps.

Thanks for all your hard work, Dave.