Disconnect settings 'changed' signal when destroying objects

This solves issue with having lots of messages logged in syslog when coming from lock screen of GNOME.

Messages looked like:
    (gnome-shell:4705): Clutter-CRITICAL **: clutter_text_get_text: assertion `CLUTTER_IS_TEXT (self)' failed

(cherry picked from commit 863e895b67a8e70809ebe3bb8aa5883e142aa00f)