Fix some bugs and usability issues in the polkit dialog:
- Make the "Authenticate" button insensitive by default until polkit asks for a password
- Make the "Authenticate" button insensitive if there's nothing in the password entry
- Also show the avatar for the root user
- Update the user name on user-changes
- Set key focus to the "Cancel" button if there's no password entry
- Fix a bug where the user avatar isn't visible on first run of the dialog