Generating D-Bus proxies is more convienent and more typesafe
than using naked GDBusProxy objects.

As a first step this commit changes the common session manager proxy to
be generated from XML. Since the generated proxy can be compatibly used
in any existing GDBusProxy calls, this commit doesn't attempt to rewrite
all the plugins to use the generated apis. That can happen in future
clean ups.

https://bugzilla.gnome.org/show_bug.cgi?id=705127