Comparing file paths can be quite expensive because the start very likely
with the same string. Sorting for size and only compare the string is less
expensive. For many algorithm we need a sort order, so making the less
operator cheaper is quite desirable.

Change-Id: I33e7abc7a65264e80376f445f8b6dcada0625ab9
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>