panelMenu: Remove the gicon parameter from addIndicator, and make private

There's only two uses of the parameter left, which can easily be added as a
separate line below. Since it's really a private interface meant for the
indicators, make it private as well so external users are less likely to
use it.

https://bugzilla.gnome.org/show_bug.cgi?id=705845

js/ui/status/bluetooth.js

@@ -20,7 +20,8 @@ const Indicator = new Lang.Class({
     _init: function() {
         this.parent();
 
-        this._indicator = this.addIndicator(new Gio.ThemedIcon({ name: 'bluetooth-active-symbolic' }));
+        this._indicator = this._addIndicator();
+        this._indicator.icon_name = 'bluetooth-active-symbolic';
 
         // The Bluetooth menu only appears when Bluetooth is in use,
         // so just statically build it with a "Turn Off" menu item.