gdm: port from libgdmgreeter to libgdm

When GDM was moved over to GDBus it dropped the libgdmgreeter
library and introduced a new libgdm library with a somewhat
different API.

The main differences in the API are:

1) open_connection is now implicit and automatic
2) conversations don't need to be started explicitly, they're
   started just-in-time when verification is requested
3) The functions are split up between the client, and new
   helper objects that correspond to the dbus interfaces
   they were generated from (one for user verification,
   one for greeter specific operations, and a couple more
   that aren't used by gnome-shell).
4) libgdm supports reauthenticating in an already running
   session, so user switching should now affect the users
   session more like screen unlocking does.

This commit moves the shell over to the new library.

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

https://bugzilla.gnome.org/show_bug.cgi?id=676401