Added comments about Platinum Properties

I found the didl-lite xsd which has decent descriptions for some of its
elements and attributes. A lot of these map directly to Platinum
Properties so I copied a lot of the descriptions above where the
properties are being utilised. Ideally this information would be in
triple slash comments inside the Platinum Library but alas it is not.
Add shortcircuit to GetChildRecursive

Added some short circuit logic to the dlna model's GetChildRecursive
function.