Document the fact that the icon on the traitsui application can be changed


By default, traitsui applications have an icon on the top-left corner of the application (on windows). Document the fact that this icon can be changed by passing the `icon` argument on the `View`. Alternatively, to remove the icon entirely, the user can manually call the `setWindowIcon` method on the control with `None`.

This might belong in a new How-To documentation section.