extensionPrefs: Fix extension list width

ScrolledWindow changed its allocation behavior, and the extension
list only takes up the minimum width rather than the available
width as intended. To get the previous behavior back, we need
to set the newly added :propagate-natural-width property ...

https://bugzilla.gnome.org/show_bug.cgi?id=771391

js/extensionPrefs/main.js

@@ -152,6 +152,7 @@ const Application = new Lang.Class({
         let scroll = new Gtk.ScrolledWindow({ hscrollbar_policy: Gtk.PolicyType.NEVER,
                                               shadow_type: Gtk.ShadowType.IN,
                                               halign: Gtk.Align.CENTER,
+                                              propagate_natural_width: true,
                                               margin: 18 });
         this._window.add(scroll);