Add progress bar to lougout dialog and make the timeout configurable

Progress bar from https://github.com/mate-desktop/mate-session-manager/commit/851092258498b7a8f526726d117a78c12fbf5513

![screenshot from 2014-07-24 12 49 45](https://cloud.githubusercontent.com/assets/1816681/3693091/4cabfa06-1363-11e4-9eb7-b9aa82f13d81.png)