connexion window (in administration menu) is not optimal

[thibaut bethune]

If i want Ubuntu to start (and shut down) in silence, i have to go to
to System>Preferences, sound entry the,n sound tab in order to
desactivate sounds.

But i also have to go System>administration, connexion window entry
(with sudo), accessibily tab to desactivate sound

from user point of view it makes non sense :
- to have to enter a password to desactivate a sound
- to have 2 differents menu entries to tweak (furthermore : one is in
preferences menu, the other one is in administration menu)
- to search in accessibilty tab the option to desactivate this specific sound

Also, GDM themes should be configurable from System>Preferences, Appearance

https://bugs.launchpad.net/ubuntu/+bug/153434