filechooserutils: add fallback name to themed icons

Instead of using the misleading gtk_icon_theme_has_gicon, which for non-themed simply assumes they exist without even trying to check, and for themed icons ends up taking locks to do unnecessary cache validation that also gets done when we actually resolve the thing later, simply add the fallback name to a themed icons list — providing the same result as the existing code

More broadly we may want to consider deprecating gtk_icon_theme_has_gicon, I also wonder if perhaps GIO should include text-x-generic as a default icon.