extensionPrefs: Work around xgettext bug

xgettext is thrown off by the combination of backticks and slashes,
which makes template string a fun trap to fall into. Just use
String.format() for now ...

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/383

js/extensionPrefs/main.js

@@ -118,7 +118,7 @@ var Application = class {
         scroll.add(box);
 
         let label = new Gtk.Label({
-            label: `<span size="x-large">${_("Something’s gone wrong")}</span>`,
+            label: '<span size="x-large">%s</span>'.format(_("Something’s gone wrong")),
             use_markup: true
         });
         label.get_style_context().add_class(Gtk.STYLE_CLASS_DIM_LABEL);