Add a PolicyKit authentication agent

A PolicyKit Authentication Agent is a construct used to authenticate
one or more identities. See the PolicyKit documentation for more
details on authentication agents and how PolicyKit works:

    http://hal.freedesktop.org/docs/polkit/

Since gjs does not support subclassing a GObject class from Javascript
code, we bring in a native class to bridge the VFuncs to GObject
signals. Additionally, this native class also queues up authentication
requests so the user of the native class only has to deal with a
single outstanding request at any one time.

The file js/ui/polkitAuthenticationAgent.js introduces a singleton
that listens for authentication requests via the native class. This
singleton uses the PolkitAgent machinery to do the actual heavy-weight
lifting required for authentication (essentially a PAM conversation).

We currently don't allow the user to pick the identity to be
authenticated.

https://bugzilla.gnome.org/show_bug.cgi?id=642886

Signed-off-by: David Ze...