PopupMenu: redo keynav using St.FocusManager

Each menu is a focus manager group, but there is also some explicit
focus handling between non-hierarchically-related widgets. Eg, to move
between menus, or from a menubutton into its menu.

https://bugzilla.gnome.org/show_bug.cgi?id=621671

js/ui/panel.js

@@ -743,8 +743,9 @@ Panel.prototype = {
         /* Translators: If there is no suitable word for "Activities" in your language, you can use the word for "Overview". */
         let label = new St.Label({ text: _("Activities") });
         this.button = new St.Clickable({ name: 'panelActivities',
-                                          style_class: 'panel-button',
-                                          reactive: true });
+                                         style_class: 'panel-button',
+                                         reactive: true,
+                                         can_focus: true });
         this.button.set_child(label);
 
         this._leftBox.add(this.button);