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/volume.js

@@ -301,7 +301,7 @@ const Indicator = new Lang.Class({
     _init: function() {
         this.parent();
 
-        this._primaryIndicator = this.addIndicator(null);
+        this._primaryIndicator = this._addIndicator();
 
         this._control = getMixerControl();
         this._volumeMenu = new VolumeMenu(this._control);