generate GDBus API for the chat manager

Plan is to get rid of our own code gen system in next.

https://bugzilla.gnome.org/show_bug.cgi?id=723766
chat-mgr: Use GDBus client side API

https://bugzilla.gnome.org/show_bug.cgi?id=723766
chat-mgr: use GDBus service API

We have now to use the GtkApplication bus name when calling the method as GBus
uses this bus name and not the Telepathy one which is used by dbus-glib.

https://bugzilla.gnome.org/show_bug.cgi?id=723766
extensions: remove Chat_Manager.xml

We use GDBus now.

https://bugzilla.gnome.org/show_bug.cgi?id=723766