Rewrote the DBus method call code to use the new APIs.

The GLib library now includes its own DBus bindings, and so they recommend switching to the new bindings for new applications.
This requires glib-2.0 version at least 2.26 (at the moment of writing the latest version is 2.40).
Removed the dependency of the binding library (libdbus-glib-1), updated appshell.gyp accordingly.