upnp: do a full LoadItem from thumb loaders instead of FillLibraryArt
gui: don't allow context menu recursion

Could happen if a button was directly mapped to a context menu.
Platinum: don't disregard PLT_MediaObjects only for lacking resources

This could cause us to misscount the number of media objects in a upnp
browse request, and re-request items we already had in list giving
us duplicated items in lists.