status: Port to a new SystemIndicator framework

We can't silently replace the old behavior of separate status
icons into a new system. Replace SystemStatusButton with a new
SystemIndicator class which will allow for the flexibility we
need. For now, make it a subclass of Button so that it mostly
feels the same, but we'll soon be swapping it out with a dummy
implementation that the aggregate menu will use.

I think the code cleanup here is worth it.

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

js/ui/status/system.js

@@ -42,10 +42,12 @@ const SystemdLoginSession = Gio.DBusProxy.makeProxyWrapper(SystemdLoginSessionIf
 
 const Indicator = new Lang.Class({
     Name: 'SystemIndicator',
-    Extends: PanelMenu.SystemStatusButton,
+    Extends: PanelMenu.SystemIndicator,
 
     _init: function() {
-        this.parent('system-shutdown-symbolic', _("System"));
+        this.parent();
+
+        this._indicator = this.addIndicator(new Gio.ThemedIcon({ name: 'system-shutdown-symbolic' }));
 
         this._screenSaverSettings = new Gio.Settings({ schema: SCREENSAVER_SCHEMA });
         this._lockdownSettings = new Gio.Settings({ schema: LOCKDOWN_SCHEMA });