A subsequent patch will directly use the BaseItem to query the
GFileInfo, instead of creating a GFile out of the URI. This is
required to transparently query the state of the thumbnail across
different BaseItem sub-classes that might have different thumbnail
caches.

This is a step in that direction.

It's also better to not do the same thing, parsing the SparqlCursor in
this case, in subtly different ways through different code paths.

#29