Port to new GDBus bindings in gjs

Rewrite code acquiring dbus names so that it uses GDBus, and rewrite
ShellDBus so that it is exposed on the GDBus connection. Ports of
the other objects will follow.

https://bugzilla.gnome.org/show_bug.cgi?id=648651

js/ui/main.js

@@ -1,7 +1,6 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
 const Clutter = imports.gi.Clutter;
-const DBus = imports.dbus;
 const Gdk = imports.gi.Gdk;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
@@ -163,11 +162,6 @@ function start() {
     Gio.DesktopAppInfo.set_desktop_env('GNOME');
 
     shellDBusService = new ShellDBus.GnomeShell();
-    // Force a connection now; dbus.js will do this internally
-    // if we use its name acquisition stuff but we aren't right
-    // now; to do so we'd need to convert from its async calls
-    // back into sync ones.
-    DBus.session.flush();
 
     // Ensure ShellWindowTracker and ShellAppUsage are initialized; this will
     // also initialize ShellAppSystem first.  ShellAppSystem