UPnP: Add support for GetFeatureList() and GetServiceResetToken().

We don't yet support any features so we return an empty result.

For the ResetToken I used the date at the point the MediaServer was
started as it fits the critera of being unique (never repeating) and
it will change when the backend restarts (internal state is lost).
UPnP: Implement the CONTAINER_SHORTCUT feature.