libgdmgreeter: generate implementation

GdmGreeterClient is the interface greeters use to communicate
with their respective slaves. It will eventually also be
useful as an interface for screensavers to do authentication.

The actual GdmGreeterClient code is a just a thin wrapper around
some libdbus calls. Something very similar can be automatically
generated using gdbus-codegen.

This commit:

    - updates the library to use the most up to date dbus
      interfaces provided by the daemon
    - replaces the hand rolled dbus code with generated code,
      but leaving the client interface to get at the generated
      objects.

Based on work by Giovanni Campagna <gcampagna@src.gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=676381