Add min-width for the indicator popup menu
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)