config.xml: Discourage use of <Icon>

<Icon> is deprecated according to the documentation, so better
tell people to use <InstallerApplicationIcon> and/or <InstallerWindowIcon>
instead.

Also moving up the settings to the same position they appear in the documentation.

Change-Id: I8fe468ba9c362f1abbeb69809eb25e2c51204b25
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>