TODO:

 * Implement the low power changed signal
 * Throw dbus errors when we fail dbus methods: org.gnome.PowerManager.NotInhibited.
 * Make strings more translatable
 * Integrate g-p-m with more modules, e.g. nautilus.
 * Give the user the ability to force an Inhibit()'d action
 * use the updated hal can_suspend_to_disk keys
