Return modem_settings from the _secrets_cb

Modem configuration uses Gtk.main_iteration() in a loop waiting for the
asynchronous call to get_secrets to return. This causes a problem with
the GLib.idle_add call in the gui to set the WATCH cursor (See #3800).

This patch uses a callback in the _secrets_cb (and _secrets_err_cb) to
return the settings, thus avoiding the need for the loop.
remove unused import of Gtk

Sugar Build fails because of an unused import. This patch removes the
unused import.
pep8'd sugar