viewSelector: Allow to start keynav using the down arrow

As the search entry acts like it had keyboard focus, it seems
logical to use the down arrow to move focus to the active page.

https://bugzilla.gnome.org/show_bug.cgi?id=682109

js/ui/viewSelector.js

@@ -247,7 +247,7 @@ const ViewSelector = new Lang.Class({
                    (symbol == Clutter.BackSpace && this.active)) {
             this.startSearch(event);
         } else if (!this.active) {
-            if (symbol == Clutter.Tab) {
+            if (symbol == Clutter.Tab || symbol == Clutter.Down) {
                 this._activePage.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
                 return true;
             } else if (symbol == Clutter.ISO_Left_Tab) {