Add privacy policy link for WinRT

For apps which use the network, the Windows Store requires that a privacy
policy link appear in the Settings Charm. As there is no Qt API for this,
add some platform-specific code to provide the policy link.

Change-Id: Iac7715ace86cf8b414d102eb724678d0ffaf2452
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>