panel: Add keyboard shortcut to open app menu

With the application menu now being more than a stub, it has
become a much more interesting target, so add a keyboard shortcut
to open it directly.
This should also ease some of the pain for focus-follows-mouse users.

https://bugzilla.gnome.org/show_bug.cgi?id=672909

js/ui/panel.js

@@ -4,6 +4,7 @@ const Cairo = imports.cairo;
 const Clutter = imports.gi.Clutter;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
+const Gtk = imports.gi.Gtk;
 const Lang = imports.lang;
 const Mainloop = imports.mainloop;
 const Meta = imports.gi.Meta;
@@ -1112,6 +1113,15 @@ const Panel = new Lang.Class({
         return true;
     },
 
+    openAppMenu: function() {
+        let menu = this._appMenu.menu;
+        if (Main.overview.visible || menu.isOpen)
+          return;
+
+        menu.open();
+        menu.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
+    },
+
     startStatusArea: function() {
         for (let i = 0; i < this._status_area_order.length; i++) {
             let role = this._status_area_order[i];