Only do a fast pre-sorting when "Sort by Name" is used

This is a follow-up to commit 0e9f4a398735cfc19ae783d2ab054d2400d95416,
which tries to speed up sorting the items naturally by their name using
the idea that a fast non-natural pre-sorting already sorts the items
mostly correctly and thus reduces the number of expensive natural
comparisons.

This change only makes sense if the view is really sorted by "Name". In
other cases, the pre-sorting will most likely not be useful.

Thanks to Christoph Feck for pointing this out!