It is not idiomatic for strings to transfer ownership when passed as
input arguments to functions, unless the function is explitly named
as such. eg., "take" functions.

Transferring the ownership of the SPARQL string to the PhotosQuery
struct did afford us some conveniences. eg., not having to free the
string. However, in a following patch, PhotosQuery is going to be
converted into a GObject. It will be even more non-idiomatic to
retain the current behaviour, and a future port to g_auto* will take
care of the extra lines of code needed to free the string.

That leaves the possibility of memory fragmentation caused by the
extra copy. However, one can't say if that's a real issue without
actual measurements. We don't really create that many PhotosQuery
instances. In any case, this will be assuaged in the following patch
where PhotosQuery instances will be made reference counted. That will
remove the need to copy the sparql and tag fields, and reduce
fragmentation.