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 Zeuthen <davidz@redhat.com>

js/ui/main.js

@@ -23,6 +23,7 @@ const _ = Gettext.gettext;
 const Chrome = imports.ui.chrome;
 const CtrlAltTab = imports.ui.ctrlAltTab;
 const EndSessionDialog = imports.ui.endSessionDialog;
+const PolkitAuthenticationAgent = imports.ui.polkitAuthenticationAgent;
 const Environment = imports.ui.environment;
 const ExtensionSystem = imports.ui.extensionSystem;
 const MessageTray = imports.ui.messageTray;
@@ -183,6 +184,9 @@ function start() {
     // initiate logouts.
     EndSessionDialog.init();
 
+    // Attempt to become a PolicyKit authentication agent
+    PolkitAuthenticationAgent.init()
+
     global.gdk_screen.connect('monitors-changed', _relayout);
 
     ExtensionSystem.init();