The GTK+ module is designed to be loaded using gnome-settings-daemon by the
gnome-packagekit package.

It can however be launched for testing using:

GTK_MODULES="$GTK_MODULES:/usr/lib64/gtk-3.0/modules/libpk-gtk-module.so" application

The module installs a custom default pangocairo font map during gtk_init().
Pango will then call back with any languages which need installing, and these
are queued up. In an idle callback these are emitted as an asyncronous D-BUS
method to the session PackageKit InstallFontconfigResources() method.

If configured to do so, this will prompt the user to install new fonts.

Notes:

 * At the moment the window XID is not detected correctly, which means focus
   stealing prevention may not work properly.  We're working on this.
 * gnome-packagekit 2.17.2 is needed for the InstallFontconfigResources method.