panel: Add keybinding mode for top bar popups and use it

Allow some keybindings to still work while a top bar menu is open
by assigning it a keybinding mode.

https://bugzilla.gnome.org/show_bug.cgi?id=698938

js/ui/panel.js

@@ -923,7 +923,7 @@ const Panel = new Lang.Class({
 
         this.statusArea = {};
 
-        this.menuManager = new PopupMenu.PopupMenuManager(this);
+        this.menuManager = new PopupMenu.PopupMenuManager(this, { keybindingMode: Shell.KeyBindingMode.TOPBAR_POPUP });
 
         this._leftBox = new St.BoxLayout({ name: 'panelLeft' });
         this.actor.add_actor(this._leftBox);