popupMenu: Use new convenience method for settings

All the system status menus in the panel offer a
menu item to jump to a relevant part of the
control-center.

This means each status icon has the same, or nearly the
same bit of code to:

- Add a new "action" menu item and listen for its activation.
- Hide the overview if it's showing when the menu item is activated
- Find the relevant control-center panel from its desktop file
- Launch the control-center to the relevant panel

This commit consolidates all those details in a new method,
addSettingsAction.  This refactoring reduces code duplication and
slight inconsistencies in the code resulting from that duplication.
It will also make it easier in subsequent commits to hide settings menu
items when the shell is used in the login screen.

https://bugzilla.gnome.org/show_bug.cgi?id=657082

js/ui/popupMenu.js

@@ -803,11 +803,28 @@ PopupMenuBase.prototype = {
     },
 
     addAction: function(title, callback) {
-        var menuItem = new PopupMenuItem(title);
+        let menuItem = new PopupMenuItem(title);
         this.addMenuItem(menuItem);
         menuItem.connect('activate', Lang.bind(this, function (menuItem, event) {
             callback(event);
         }));
+
+        return menuItem;
+    },
+
+    addSettingsAction: function(title, desktopFile) {
+        let menuItem = this.addAction(title, function() {
+                           let app = Shell.AppSystem.get_default().lookup_setting(desktopFile);
+
+                           if (!app) {
+                               log('Settings panel for desktop file ' + desktopFile + ' could not be loaded!');
+                               return;
+                           }
+
+                           Main.overview.hide();
+                           app.activate();
+                       });
+        return menuItem;
     },
 
     /**