Implement privacy settings to prevent IM spam

[diederik]

MSN has a privacy setting in the protocol. This can be used to disallow chat conversations with users which are not on your contact list.

See more at: http://msnpiki.msnfanatic.com/index.php/Notification:GettingDetails#Privacy_Settings

Stuff to do:

• add a checkbox in the settings panel for this account setting.
• this checkbox should be grayed out if you're not connected with that account.
• the proper commands should be read and sent from/to the server.