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.

gtk/gtkfilechooserutils.c

@@ -173,7 +173,7 @@ _gtk_file_chooser_delegate_get_quark (void)
 
   if (G_UNLIKELY (quark == 0))
     quark = g_quark_from_static_string ("gtk-file-chooser-delegate");
-  
+
   return quark;
 }
 
@@ -357,9 +357,9 @@ _gtk_file_chooser_label_for_file (GFile *file)
         end = p;
 
       host = g_strndup (start, end - start);
-      /* Translators: the first string is a path and the second string 
-       * is a hostname. Nautilus and the panel contain the same string 
-       * to translate. 
+      /* Translators: the first string is a path and the second string
+       * is a hostname. Nautilus and the panel contain the same string
+       * to translate.
        */
       label = g_strdup_printf (_("%1$s on %2$s"), path, host);
 
@@ -474,12 +474,14 @@ _gtk_file_info_get_icon (GFileInfo    *info,
     }
 
   icon = g_file_info_get_icon (info);
-  if (icon && gtk_icon_theme_has_gicon (icon_theme, icon))
-    return g_object_ref (icon);
 
   /* Use general fallback for all files without icon */
-  icon = g_themed_icon_new ("text-x-generic");
-  return icon;
+  if (!icon)
+    return g_themed_icon_new ("text-x-generic");
+  else if (G_IS_THEMED_ICON (icon))
+    g_themed_icon_append_name (G_THEMED_ICON (icon), "text-x-generic");
+
+  return g_object_ref (icon);
 }
 
 GFile *