Sort in items in two stages to speed up natural sorting

Sort the items in a folder first according to their name, without doing
a natural/locale-aware sorting. This is very fast, but the order of the
items is then already close to the final order in most cases.

The number of expensive natural comparisons required to sort the items
is thus greatly reduced.

In my experiments with a folder with 100,000 items, the time required
to sort the files was reduced by 63% with this patch.

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