gdm-shell: Adapt to gnome-shell changes

gnome-shell no longer accepts a --gdm-mode commandline option.
Instead, it now accepts --mode=gdm.
Add DBus utilities to create private servers

Consolidates code that creates a private DBus server, including
generating the address and setting up the required message bus
interfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port GdmGreeterServer to GDBus

GdmGreeterServer will soon see changes to allow multiple interactions
at the same time, and porting to GDBus is a prerequisite to make
the code more manageable.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Use GDBus for GdmSettings

Second patch in the GDBus port, this is primarily to ensure that
everything the slave expose is in what GIO thinks is the system bus.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port GdmFactorySlave to GDBus

Last commit moved global bus initialization to load GDBus instead
of dbus-glib, so this commit ensures that all traffic on the system
bus is through GIO.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Continue porting GdmFactorySlave / GdmProductSlave to GDBus

This commit tackles the GdmSession infrastructure, trying to
consolidate GdmSessionRelay and GdmSessionDirect into a single
generated class. For now, only GdmSessionRelay is ported (both
client and server side).

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Refactor GdmSessionDirect to use GDBus and generated bindings

Since GdmSessionRelay went away in the last commit, GdmSessionDirect
was the only class implementing GdmSession, so drop the latter
and use GdmSessionDirect in slaves. Also, refactor it to be an
abstraction over multiple GdmDBusSession instead of talking DBus
directly.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port CkConnector to GDBus

Port the CkConnector glue code to GLib and GDBus.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port GdmSessionWorker to GDBus

This finishes the GdmSession saga, bringing the last piece inline
with the generated code.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port GdmDisplay, GdmXdmcpSlave and friends to GDBus

This is one big commit because it uses generated code both
in the daemon and in the slaves, so we need to port both at the
same time.

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port the gdm-binary daemon to GDBus

Use GDBusObjectManager for enumerating displays, instead of a
hand rolled interface. Also, fix a bunch of present but not
really working interfaces (slave bus name watching, display-added/removed
signals).

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Finish porting the daemon to GDBus

The daemon, all the slaves, and the session worker now all use
GDBus exclusively. Dependency on dbus-glib has been removed (but
it still remains for the GUI parts)

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Port gdmflexiserver to GDBus

One less dependency on dbus-glib, yay!

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Fix distcheck

Add missing xml files to EXTRA_DIST and generated glue code to
BUILT_SOURCES.
Update .gitignore

https://bugzilla.gnome.org/show_bug.cgi?id=622888
Don't load properties when creating proxies

No interface of ours uses properties, and this avoids a bunch
of dbus failures in syslog.
Add an interface for acting as a greeter from the session

Add a new method to org.gnome.DisplayManager.Display that allows
opening a private connection to the appropriate slave, which in
turns exports a reduced form of the GdmGreeterServer interface,
on a separate GdmSessionDirect.

https://bugzilla.gnome.org/show_bug.cgi?id=676381
Use generated GDBus for libgdmgreeter

GdmGreeterClient was just a wrapper over the DBus interface, and
was in fact very similar to the autogenerated bindings. Bite the
bullet and replace GdmGreeterClient with generated GdmGreeterServer.
Also add two convenience functions for creating GdmGreeterServers,
one for actual greeters and one for session components that want
to use the greeter interface.

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