status: use Shell.AppSystem to launch settings panels

Different methos are being used to launch the control-center panels of each
status icon. Standarize on Shell.AppSystem.
This also fixes the network icon using a non-existant Util.spawnDesktop()
method.

Bug #645091

js/ui/status/bluetooth.js

@@ -93,7 +93,8 @@ Indicator.prototype = {
         this._updateFullMenu();
 
         this.menu.addAction(_("Bluetooth Settings"), function() {
-            GLib.spawn_command_line_async('gnome-control-center bluetooth');
+            let app = Shell.AppSystem.get_default().get_app('bluetooth-properties.desktop');
+            app.activate(-1);
         });
 
         this._applet.connect('pincode-request', Lang.bind(this, this._pinRequest));